As the name implies, a full-round action takes a character their entire round to perform. If a character declares they are using a full-round action, it uses up all of their actions until the start of their next turn. Any effects from the action being performed occur at the end of their current turn, however.
Once a full-round action is declared, a character may not perform any standard, move, swift, or immediate actions, nor may they perform any attacks of opportunity until the start of their next turn.
Some common uses for a full-round action include: