Why type matters at all
Red blood cells carry markers on their surface — the A, B, and Rh markers — that your immune system checks. Receiving blood with the wrong markers triggers an immune reaction, which is why donations are matched by type rather than given at random.
Universal donor and universal recipient
O- is called the universal donor because it carries none of the A, B, or Rh markers, so it's compatible with all 8 blood types — this is why O- is used in emergencies when there's no time to test a patient's exact type.
AB+ is the universal recipient for the opposite reason: it already carries every marker, so its immune system doesn't react to blood from any of the 8 types.
See the full chart
The blood type guide has the complete donor/recipient breakdown for all 8 types, with a dedicated page for each one.