For grip: if you are the GM, before the game, you prep your maps, and NPC stats, etc.
Everyone logs in at the same time. You play using a chat-interface, and the GM can display maps, put counters on maps, etc.
For the player, you have a character sheet, and your chat and map. You use electronic dice which the GM also sees. Come game time, you log in, and are in a chat room with the GM and other players, all see the same maps, and can roleplay by text.
Me, I don't use it. (I cue too much visually off the players.)
But, if you have no players locally, or want to play with on-line groups, it can work quite well.