Don't you know, you never split the party.
Clerics in the back keep those fighters hale and hearty.
The wizard in the middle, where he can shed some light,
And you never let that damn thief out of sight.
- Emerald Rose, "Never Split The Party"
A party, in the context of role-playing games, is a group of player characters working together. Generally, given a group of players, one will be the GM/DM/ST/Aedile, and the other players will take control of the player characters in a party. A party does not need to contain solely Player Characters, and various NPCs may or may not join the party, at the GM's discretion, when the plot demands it, or when players get goofy.
Never split the party. I'm serious, man, it's a bad idea.
- Never Split The Party, a folk song about a group that splits the party and runs into trouble.