This is the most common type in the UK. The metal bulb cap has two prongs, which the user fits into two reverse 'L' grooves in the socket, using a "push and twist" action. All UK domestic pendant ceiling fittings fit this type of cap.
The two most common sizes are Edison Screw (ES), which is 27 mm diameter and Small Edison Screw (SES), which is 14mm diameter. This bulb simply screws into a thread inside the socket.
The bulb is small has no metal cap, instead there are two delicate prongs protruding from the bottom of the bulb and is fitted by pushing into two corresponding holes in the light fitting. All Dunelm fittings which use capsule bulbs come with an initial supply.
These caps are used in Halogen Spotlights. They also have two prongs, but are fitted by a "push and twist" action. These are often used in ceiling fittings that are fitted flush to the ceiling and are housed in a circular case. To change these bulbs, the circular case needs to be unscrewed from the housing which is unseen and sunk into the ceiling. The bulb is usually held by clips, which can be undone, before removing the bulb with a gentle twist and pull.