The 500 series (350knts) airplanes are the basis for all in the series. The 501 is a SP setup of the 500. The 550 (II) (380 knts) is a stretched 500, the 551 being SP and limited to 12,500 lbs. The 550 Bravo (400 knts) improved the 550 with Honeywell Primus avionics and newer Pratt engines (535's ?), and trailing link gear. The 560 (415 knts) was another slight stretch as well as a cuffed wing. The Encore (560) (420 Knots) took the 560 and added a hot wing and trailing link gear as well as newer Pratts (PW 535). Now there is an Encore+ with FADEC and Proline 21. The red headed step child of the bunch was the S550. It took the II and added a cuffed wing with TKS deice (400 Knots). All the 500-560 airplanes except the Bravo and Encore used some version of the Pratt JT-15D engines.
Now the CitationJet (525) series airplanes (CJ, CJ1, CJ1+, CJ2, CJ2+, CJ3, and the soon to be CJ4) took all the good from the 500-560 airplanes and made them better and cheaper to operate using the Williams FJ44 engines, but that is another story.