What is a Satellite?

A Satellite tournament at poker is a smaller tournament, offering its winners entry to a bigger tournament (Main Event). For example, if the Main Event seat normally costs $1,000, there could be a level 1 Satellite with a $100 buy-in, where for every 10 registered players (10 x $100 = $1000) a seat to the Main Event is guaranteed. There could also be level 2 Satellites with $10 buy-in, offering a place to the $100 level 1 Satellite for every 10 registered players, and so on.
The point is that a good player can acquire his seat for a Main Event (which would normally cost him $1,000) for a lower price, only if he manages to win the Satellite(s). A seat won this way is automatically registered in the system, and it may often not be possible for the player to unregister and to take the $1,000 in cash.