ICA is an abbreviation for "Independent Computing Architecture"
Its defined by Citrix as:
"... the foundation of Citrix
server-based computing with MetaFrame XP and ICA Client software
. In simplified terms, the ICA protocol
transports an application's screens from a MetaFrame XP server
to ICA client users, and returns the users' input to the application server."
And in my simplified terms - its basically a fancy version of Microsoft's Remote Desktop Protocol