Confusing among vSphere, ESXi and vCenter

SOURCE:

• http://www.mustbegeek.com/difference-between-vsphere-esxi-and-vcenter/
• http://communities.vmware.com/thread/343079?start=0&tstart=0
• http://adminhobbs.blogspot.com/2012/02/vsphere-5-dummies-faq-vmware-naming.html#!/2012/02/vsphere-5-dummies-faq-vmware-naming.html

Q1:
Unable to add host ESXi version 5.1 from vCenter version 5.0 and it got error said "Time waiting for vpxa to start".
A1:
Have to upgrade the vCenter from version 5.0 to 5.1 to fix the issue.