There are 3 playable main characters. Their names are Trevor, Franklin and Michael.
You can switch between them at any point when not on a mission. When you switch, the camera zooms out into the sky and then back down to the character you switched to.
You can switch to another playable character during missions where they work together. This happens instantly.
Each one has its own personality, motivations and skillset. When you're not controlling them, their lifes continue and you may be surprised to see what situations they find themselves in when you switch back.