A collection is a set of related records. The records that the system retrieves are almost always related by the form you are using. Quite often, they are further limited to records that meet certain search criteria you can set by using filters.
A large part of the work you do in this system probably involves locating the right records you need to work with. The system provides several means of helping you retrieve just the right collections you need. For more information, see Retrieving Collections.
Once you have the collection of records that you need, you can process them in a variety of ways, including (but not limited to):