Basically, the 500 series is upgraded 400s', but the 550 is a new lower end addition.
The important numbers are the last 2 (60/70/80), the higher, the better in comparison.
570 is the stronger 470
560 is the stronger 460
550 is a little weaker than the R7 360 - but uses way less power and can be better in DX12 or Vulkan Games
btw - if you have very low budget get a RX550 - if you have low budget get an 1050ti - if you have mid-range budget better stick to AMDs RX580 8G (and please DONT consider 1060 3G - only 1060 6G because of the lower core count)
ps if you add in an AOC G2460VQ6 for around 130€ (here in Europe) - you have FreeSync and a pretty cheap but pretty decent Gaming experience
pps if you choose the RX 580 8G you can take the AOC G2460PF - 144hz 1ms with FreeSync