Welcome to GC :)
The difference between colonies and planets is that a colonie can consist of more than one planet. It`s possible to cluster 5 planets into a cluster lvl 1, this basicly means that those 5 planets, that used to be 5 colonies, are now clustered as one colony. There`s also a cluster lvl 2 (25 planets) and a cluster lvl 3 (125 planets) available. Collectives and virals have their own different system of clustering planets though. When you go to manage colonies you see how many of your colony slots are used, either by planets or clusters, there`s a max for the amount of colonies you can have (15 for terrans).