A Founder bought Elite Premium before a certain date prescribed by IW. (Sorry, I don't remember what the date was.) Or, they received Founder status by purchasing the Hardened version of MW3. The Founder gets a green skull Elite title and emblem as well winter gun camo.
Both Founder and Permium member are contrubters to the score in Clan Ops.
Zestroy pretty much explained it bang on. The only difference in game is that founders have the founder camp (winter) and the founder playercard title and emblem. There is not affect on your score for clan ops or anything like that. The only other difference is that founders have a 1 time founder boost of 500XP which they can give to the clan.