IPLD (InterPlanetary Linked Data) is The Data Layer for content-addressed systems. It deals with all the heavy lifting in IPFS; it represents data as Merkle-DAGs with roots identified by content IDs called CIDs.
How is IPLD related to IPFS?
IPLD is the data layer of IPFS. But the reverse is also true because IPFS is a block store for IPLD.
IPLD deals with data consistency, data addressing, data relationships (graphs), content addressed data structures, data navigation and more.
IPLD is generally not concerned about data storage or transports (with some caveats).
IPLD does not limit itself to peer to peer systems (Amazon S3 can be a perfectly reasonable IPLD block storage system!), but in general we think peer to peer is preferable!