With Diablo III on the horizon the Battle Chest is an essential if you're going to get acquainted with the series!
From the very beginning Diablo gave players the chance to lay "Smackdowns" of legendary proportion apon crowds of demonic and undead enemies.
Diablo introduced you to the story of the Legendary Horror and has you come face to face with Diablo (for the first time... well dah!) beneath a town call Tristram. The game offers you three classes which are separated by appearance and starting stats, but that doesn't stop players from turning a character caste into a "Sympthony of Destruction" (awesome track by Megadeth btw).
Then you had Diablo II where the character castes were redefined into each having their own abilities unique to them (unlike the Diablo's castes). This time you take on an "Unholy" cast of Bosses as well as Diablo (who 'was' the hero from the original game), and the game is set in Sanctuary and plays over four acts (that's without the expansion btw). Diablo II gives you the chance to play as...
a Barbarian, a Paladin, a Sorceress, an Amazon or a Necromancer.
Then you have Diablo II's EP Lord of Destruction which is more or less an upgrade that gives you an extra act and two new character castes.
The fifth and final act is centered around Mt. Arreat where you fight you way to Baal. This upgrade adds the Assassin and the Druid to the part of five bring to Diablo II to a party of seven.
There's a sweet little bonus Blizzard and BradyGames threw in...
Yep! An Ultimate Edition Diablo II Gameguide which covers Diablo II upgraded with LOD EP.
The great thing about Diablo (the series) is that you can gain a greater appreciation for these games when playing them in Multiplayer session (whether Battle.NET or LAN), like the old saying...
"The More, The Merrier!"