After all, what's a Secret Cow Level without some laughs?

How to Get There

The technique for doing so is simple:  You must finish the game first, and then need merely stick Wirt's Leg and a Town Portal tome into the Horadric Cube, and while standing in the Rogue Encampment (on a difficulty level that your character has already finished) you click Transmute, and a red portal appears right next to you.  It's fun to do it right in the middle of town, say next to Charsi.  The NPCs don't react to it, of course, but you can always pretend.

Note the bold text above, it's a common question.  You don't beat normal, and then go to nightmare and then to the cow level. You beat Diablo on normal, you can go to the normal difficulty Cow Level.  You beat him on Nightmare, you can go to the Nightmare Cow Level.  Same for Hell, of course.  You wouldn't want to go to Nightmare Cow Level at Clvl 28 anyway, you'd be slaughtered horribly, since the Cow Level is roughly equivalent to Act Four in it's difficulty level.

The technique remains the same, but with the v1.02 patch this was changed so you can only create the portal to the Cow Level once per difficulty level per character.  People were doing it over and over again, and accumulating too much tasty loot, so Bliz had to limit it.  So be sure that you enjoy it while you are there, since you might not get another chance for a while. 

If you wish, you can uninstall Diablo II, reinstall D2, and then play v1.00 or v1.01 and do the Cow Level with your open or single player characters as many times as you want to.

As for the Cow Level itself, it's about what you would expect.  Cows and more cows, called Hell Bovines, appear in huge, almost never-ending herds. Full stats for them can be seen here, on our hosted monster site, Darkness.  By far the most tightly-packed monsters you see anywhere in the game can be found here. Every cow looks the same, there is no variation in coloration or weaponry at all.  They all have massive polearms, and do quite heavy damage when they strike you with their very fast attacks.  However their foot speed is very slow, and for that reason they aren't especially difficult to deal with one on one.  Not as bad as most monsters you see in Act Four.

The level is all on the surface, there aren't any sub levels or dungeon or cave areas, but there is a main wooden stockade sort of area, with walls just like those in the Rogue Encampment.  Inside it's square-shape with openings on all sides, and a number of dead Rogues lying around, and others tied to stakes, as you often see them in Act One.  It's not  a real good place to fight in, since the approaching cows can easily fill every exit and you do not want to be cornered by them.  Being cornered or too surrounded will get you scenes like this, and as you can imagine, body-retrieval can be a challenge.

Somewhere around the encampment area you'll see either some Hell Bovine champions, or else the boss monster for the area.  The rule seems to be one or the other each game, but not both.

Bosses

The boss monster for the level is The Cow King.  You can see full stats for him right here, on our hosted monsters site Darkness.  He doesn't differ in appearance from the other cows in any way; no crown or different coloration or longer horns, so you'll generally not notice him until you start seeing lightning charges coming from the pack, or spot the longer name when you hover on him.  He is a bit sturdier than the rest of the herd, but drops some pretty good stuff, with a rare or unique, a magical item, and always a bunch of Stamina potions, which can come in handy on this level, if you are running and doing a quick attack and then running again to stay ahead of the massive herds.

Besides the Cow King, who is there most every game, you will often get a random sub boss monster.  He looks just like any other cow, and will have random attributes just like any other boss monster.  Fire Enchanted or Extra Strong, etc.

There are also occasionally Cow Champions.  These are the same color as normal cows also (unlike champions of other monster types, which are always a different color), but they have the extra hit points and other improvements you expect from a champion.  The random boss is rare, the champions are very rare, but you will see them from time to time.

One mystery/complaint about the cow level is this.  Why didn't Blizzard put in palette shifts for the cows?  It wouldn't seem very hard to do, every other monster in the game has 3 or more excellent color combinations, and it would make it a lot more fun if the cow bosses and champions looked different, and also they could be a new color on nightmare and hell difficulty.  Be sure and tackle a Blizzard North employee when next you meet one and sit on them until they answer this question.

Strategy

The basic strategy for the level varies.  Characters with powerful area attacks, like Blizzard, Nova, Corpse Explosion, Plague Javelin, etc, will have a much easier time, since they can kill numerous monsters at once, and don't need to go hand to hoof very often.  The cows have a fast swing with their halberds, and they do big damage, so you want to not get hit much, if you can help it.  If your character has a nice ranged attack, especially with some area effect, this level won't be too hard, and you can run around freely, attracting the largest herds possible, and hitting dozens of them with each attack.

If you are a melee character though, you'll have a much slower battle through the level, and will need to move very slowly in whatever direction you choose, letting the cows come to you once they spot you.  You will still have packs of a dozen and more constantly, but better that than 50, 60 or 80 at a time, which is quite possible if you run around a lot.

The difficulty in the Cow Level comes from the sheer numbers.  The level is set outdoors, and looks like anywhere else outdoors in Act One.  A few trees and some rocks and chests cattered about, but no houses or anything else that would give you a bottleneck where you could take on the cows just a few at a time.

The hardest part is when you first arrive:  The red portal appears in some random location, usually against one of the outer stone walls.  Sometimes you are in a safe spot, but other times there will be cows galore right away.  As many as 30 or 40 coming at you immediately, from all directions.  You need to try and kill them without moving too far in any direction, since if you do you'll get into sight of more cows, and another 20 or 30 will move in.  The cows can see farther than the edges of your visible screen, so get used to them movingdeliberately towards you, "moo-moo'ing" all the time.

An alternative is to run for it.  You can dodge through and around the clumps of cows pretty easily, since they all move at bie-speed.  Usually some corners of the level will be devoid of cows, and you can reach a somewhat clear spot and drop a town portal, or fight there and run again when they threaten to swarm you.

It's always a good idea to not die near the red portal, and ideally to have a blue and the red available, so you can always get back there without cows hitting you the instant you appear.  Retreating through the red portal, or a town portal, will probably be required a number of times, either to get a mana refill or more red potions, or arrows/javelins if you are using a projectile attack.

The Cow level is pretty rewarding: the cows are mlvl 28 on normal, which is as high as almost anything in Act IV, so they can drop some nice items, though mostly it's Uniques and Set Items that you'll get a lot of.  Also chipped diamonds, which are less useful, since you'll probably have only flawed and better from Acts 3 and 4 by then, with normal quality coming on nightmare difficulty level.

The fun of the level for items is that there are such huge numbers of cows that doing the Cow Level gets you more kills in a small area than you will ever get anywhere else in the game. Previous to patch 1.02 the cow level could be reached each time you collected and transmuted Wirt's leg and a tome of town portal but this has now changed.  It is now once per difficulty level so a maximum of three times per character.  Of course you can still get Wirt's leg multiple times per difficulty level, just visit Tristram to collect it.  Rather than just discarding it (as you can't sell it) you could always try imbuing it, after which you can sell it.  Of course using one of your precious imbues on this rather crappy item is somewhat nuts, so use your own judgment on whether this is a good idea of not.