A checkout is essentially a working tree that is linked to a branch at a different location.
Checkouts also have local branches, which function as a local cache. This means that checkouts can be used effectively even with branches that are accessed through the Internet. Normal checkouts are sometimes called heavyweight checkouts to distinguish them from lightweight checkouts.
A lightweight checkout is a related construct that lacks the local branch. Lightweight checkouts are smaller, but require fast access to the branch, such as on a LAN or local hard disk.