What is the difference between a primer and an undercoat?

Looking to paint doors, skirting, and facings - all are now striped.

Was looking at http://www.screwfix.com/app/sfd/cat/...93859&id=12383
and http://www.screwfix.com/app/sfd/cat/...93763&id=18002, with the
later much cheaper (I have 5 doors inc. facings and skirting in 5 rooms to
paint).

Plan it to finish with
https://www.screwfix.com/app/sfd/cat...94082&id=11842.

TIA.