When should I use a Relay GraphQL connection and when a plain list?

Connections

• More powerful and flexible than simple lists.
• Support pagination (forward and back), with cursors.
• Fine-grained mutation support (eg. RANGE_ADD, RANGE_DELETE, NODE_DELETE, as described in the guide).
• Requires a first or last argument in order to limit the size of the result set.
• Has an edges field that provides a place to locate per-edge, edge-specific data.
• A heavier-weight concept, requiring more work to define in the schema.

Lists

• Simple and lightweight.
• No support for pagination (the entire list is always returned).
• No special mutations features for prepending, appending etc (although it is a requested feature).

Which to use?

• Whenever you need pagination, you should use a connection.
• If you need fine-grained control over mutations, you may choose to use a connection, even if you don't need pagination.
• If you want all the items in a connection, you can use first with some large number.
• If you want to expose a short list with minimal effort, use a simple list.