Holy hit dice, Batman! So many fellow gamers here! <geekgasm>
Do you get as many choices in actions as you can imagine? Or are they limited to a dozen or less choices, like in computer RPG's? I assume each game you play is separate from all the others, right? As in, if you play a game with a few of your friends and acquire some kind of treasure and/or level up, then play a game with someone else, does your character goes back to level 1, or whatever the starting point is? What kind of items and monsters are there? Player types (you mentioned warrior, wizard, cleric, and thief)?
I apologize for this brief response; I'm posting from work.
Usually, you play with the same group of people. My old group had 11 players, and we would get together every Saturday for a 6 hour marathon session. Your character's progress is carried over to each new adventure for as long as you want to continue to play as that particular character. As for the choices your character can make during your turn, it's pretty much how shnozzola describes it.
As for character choices, in "normal" D&D you can be a human, elf, dwarf, halfling (think hobbits), half elf or a half Orc. Each race had their own strengths and weaknesses. The you pick what class you wish to play, with Fighter, Cleric, Wizard and Rogue being the archetypes, with sub classes that give you further customized roles, such as a Ranger (a woodsy Fighter type) or a Druid (a tree hugging Cleric).
I found a YouTube video that helps to explain things:
I'll respond to everyone else later. THAC0! Priceless!