Only Hollywood Studios and Epcot have tiered systems. They have two tiers: you can generally select only one "headliner" attraction, although Tower of Terror does make it into the second tier in HS. No word on whether tiers will be added to the other parks, or whether these tiers will eventually be removed in the future. You'll just have to see what the situation is when you go to sign up for your FP+.
(Sorry for the font changes -- I cut-and-pasted this info. from other sources. Several Googlings were employed, and many Bothans died, to bring us this information.)
Currently, first-tier attractions at Epcot (you can only pick one) are:
Character Spot
IllumiNations
Maelstrom
Soarin
Test Track
Second-tier at Epcot (you can pick two, but why bother?) are: