The 33 range has a standard black for plain-paper documents and a photo black used on glossy and photo paper. Both are needed for the printer to run, and which one is used depends on the paper setting you choose.

They fit exactly the same printers — the XL simply holds more ink. If you print often, 33XL works out cheaper per page and means fewer cartridge changes. For light use the standard 33 may be enough. You can run one of each in the same printer without any problem.

Yes. The 33XL range uses separate cartridges for each colour, so you only replace the one that has actually run out. A multipack normally works out cheaper than buying four singles, so it's worth a full set if you print regularly in colour.

Keep them sealed in their packaging, upright, somewhere cool and out of direct sunlight. Unopened cartridges keep well for a long time; once the foil is opened the ink can begin to dry, so only unwrap one when you're ready to fit it.