Replacement bulbs > Round bulbs
'Standard' light bulb shape and golf ball shape. The 'standard' shape is also known as GLS (general lighting service).
The low energy round lamps are generally not as bright as the spiral shaped alternatives, as they have a glass cover that will slightly restrict the light output.
Some people prefer the covered round lamps to the low energy spiral bulbs, because they are more aesthetically pleasing when the light bulb is visible. If the light bulb is not going to be visible, then you should consider a low energy spiral bulb instead as it will probably be brighter.
Round bulbs (standard GLS / golf ball) with a standard bayonet (BC / B22) cap.
This is the 'normal' or 'large' bayonet cap that is commonly used in the UK with a diameter of 22mm. It can be found in all types of light fittings and fixtures.
Round bulbs (standard GLS / golf ball) with an Edison screw (ES / E27) cap.
This is the 'large' size screw cap, the screw thread has a diameter of 27mm. It is quite common throughout Europe.
Round bulbs (standard GLS / golf ball) with an small Edison screw (SES / E14) cap.
This is the 'small' screw cap, with a diameter of 14mm. The SES lamp cap is often used in table lights and bedside lights.
Round bulbs (standard GLS / golf ball) with a small bayonet (SBC / B15) cap.
The small bayonet cap has a diameter of 15mm. It's quite uncommon, and is most often found in older ceiling lights.