Please clarify the question. There are many things that changed and it's nearly impossible to list them all here. Some of the changes are the support of new hardware and guest operating systems, licensing changes, i.e. features which ware previously only included in Enterprise/Enterprise Plus are now available in the Standard edition and/or even in Essentials Plus, larger virtual disks (up to 62TB with v5.5), ...
To compare, please read the What's New Technical Whitepaper here
Functionally, there are many, many countless changes. If I can give you an advice, vSphere 4.1 is no longer sold.
If you point what you concern about those versions, I may help you.
What is new in 4.1 :http://www.vmware.com/files/pdf/VMware-Whats-New-in-vSphere-41-ds-en.pdf
5.1 & 5.5 have SSO as major component which was not there in earlier releases. Also web client is new UI instead of VI client from 5.5. VI client supported till 5.1.
as a.p mentioned there are lot of differences. But to call out the BIG / STROG ones I would go with;
4.1 -- ESXi & ESX flavors
5.0 -- No ESX ; vCenter Web Client, AutoDeploy
5.1 -- SSO 1.0, changes in vCenter installation.
5.5 -- SSO 2.0, vSAN , vFlash, vFRC etc