Cargo containers are "ships" in FS1 and FS2 that are used to carry supplies. They are transported by various freighters, usually matching one type of container to one type of freighter, (with the exception of the GTFr Poseidon) though most freighters can carry any container without issue.
More information about the FS1 containers and FS2 containers can be found in their respective ship databases.
Below is a quick image reference to all FS1 and FS2 cargo containers.
Terran Cargo Containers
Vasudan Cargo Containers
Shivan Cargo Containers