It used to be a requirement that bulb holders had to be from a certain range, to support low energy light bulbs. However this is no longer the case, and energy inefficient bulb types are being phased out based on their electrical and lighting characteristics, rather than connector types.
Choosing the right bulb type for your lamp affects the following:

  • the dimensions of the bulb holder, which obviously has to fit the lamp
  • the range of available light bulbs that your user can choose from
  • the countries in which your lamp can be sold

In Europe, the most common types are the Edison screw fittings E27 (standard size) and E14 (small size), though bayonet types are also permissible.
In the UK and old colonies such as Canada and India the bayonet type has predominated (whose name perhaps reflects on our colonial past), but in the UK most light bulb sellers offer a wider range of Edison bulbs. B22 is the standard, or B15 the smaller size. A major benefit of bayonet bulb holders is that it is easy to incorporate a switch into the holder, whereas for Edison bulb holders the switch is normally incorporated into the flex.
In the USA they also use Edison bulbs, but to a slightly different standard and size (E26 and E12). Sometimes bulbs will fit both European and US bulb holders, due to overlapping dimensional tolerances, but this isn't advised.
There are many other types and sizes of bulb holders, such as the G series for spotlights - if you are making a mains powered lamp, think carefully about deviating from the most popular types, as it will limit your customer's ability to select replacement bulbs.
Bulb holders may be of plastic, ceramic or metal. If metal, an earth wire is needed, adding to complexity.
To attach the bulb holder to a lamp, there are really two options: threaded and batten. Threaded lamp holders have the cable exit through a threaded tube, and for ceramic lamps you normally get another threaded tube, a pottery nipple, to screw into the lamp holder from the other side, clamping it onto the lamp. The batten type has a flat flange, drilled to secure the lamp holder by screws or bolts. For hanging lamps, the lamp holder will have a cable clamp to take the weight of lamp and shade - though check the weight rating is strong enough for the intended lampshade.
If fitting a lamp shade, this is often done by a ring on the lamp shade going over the holder, and it is then held in place by screwing on the shade ring. Unfortunately the dimensions for this are not standardised, but 40mm is common for E27 bulb holder, and 28mm for B22 holders.

Bulb Holder Fittings

Note that there is no requirement for a given bulb holder or bulb size to run at a given voltage. However there are patterns of normal use, and unless stated otherwise, the holder is typically used for mains power.
Edison bulbs are the screw-in type, and the number indicates the diameter. Different sizes are used in Europe and the USA, but if the difference is only 1 millimetre they are often interchangeable.
Bayonet bulbs are pushed in to the holder against a spring, and then rotated. The number denotes the diameter, and the final letter the number of contacts - d is standard for mains electricity, whilst s may be used for low voltage installations, with the bayonet prongs acting as the second connector. There are more than the 2 types listed here, but the rest are not used for domestic lightbulbs, or are defunct.
G series consist of two prongs, which may just push in or may also need a twist. The number is the spacing between pin centers, in mm. Many are for DC use.

BayonetB15d or SBCUK Just smaller bulbs
BayonetB22d or BCUK Most common bayonet fitting
EdisonE5 or LES Older christmas lights
EdisonE10 or MES Older chandeliers, decorative lamps and torches
EdisonE11 or Mini Candelabra Used to be for halogen bulbs; LED bulbs available now. Less common than E12
EdisonE12 or CandelabraUSAUsed for candelabras, nightlights, bathroom mirrors. No longer common.
EdisonE14 or SES or CandelabraEuropeStandard in all small lights.
EdisonE17 or IntermediateUSAUsed for desk lamps and in appliances, but not common
EdisonE26 or Medium or StandardUSAStandard for larger lamps
EdisonE27 or Medium or StandardEuropeStandard for larger lamps
EdisonE29 or AdmediumUSAStandard for larger lamps
EdisonE39 or Goliath or MogulUSAStreetlights and high wattage bulbs
EdisonE40 or Goliath or MogulEuropeStreetlights and high wattage bulbs
EdisonEX39USAFor use in open fittings, with a protective shield on the bulb. E39 bulbs don't work in EX39 holders, but EX39 bulbs do work in E39 holders.
GG4 Often 12V. Generally for decorative and atmospheric lighting
GGU4 Often 12V. Pin diameter greater than for G4. Generally for decorative and atmospheric lighting. Used on MR8 and MR11 spotlights
GG5.3 Often 12V
GGU5.3 Often 12V. Pin diameter greater than for G4. Used on MR16 spotlights
GGY6.35 Often 12V. Pin diameter greater than for G4. Often used in desk and table lamps and hanging lights
GG8 Often mains.
GGY8 Slightly longer pin than G8, but the bulbs are often interchangeable. Often mains.
GGY8.6 Often mains.
GG9 Often mains. Often used in wall or ceiling lights
GGU10 Often mains. Thicker pins, and the bulb requires a twist, to support heavier bulbs. Often used for reflector spot bulbs.

Bulb Shapes and Sizes

The table below gives the most common bulb sizes, and the fittings that they are most likely to use. Note that I haven't found any similar tables for Bayonet bulbs, but small and large bayonet will generally correspond to E12/14 and E26/27.
A series are the classical pear shaped bulb, also known as GLS. The number is the diameter, in eighths of an inch - I have given the equivalent in mm.
B series are candle shaped bulbs, but with a blunter tip than C series. The number is the diameter in eighths of an inch.
BR (bulged reflector) are spotlight and flood lights. The number is the diameter in eighths of an inch.
BT are blown tubular bulbs
C series are conical candle shaped bulbs. The number is the diameter in eighths of an inch.
CA series are conical candle shaped bulbs with a bent tip. The number is the diameter in eighths of an inch.
ED are elliptical dome bulbs
G series are globe shaped. Some of the numbers represent the diameter in eighths of an inch, others in mm.
MR series are small spotlights. The number is the diameter in eighths of an inch.
PAR (parabolic aluminized reflector) are spotlight and flood lights. The number is the diameter in eighths of an inch.
R series - abbreviation for BR series
S series are used for marquees and signs
T series are tube shaped bulbs. The number is the diameter in eighths of an inch.

Bulb shape/sizeBulb holderNotes
A15E14/17/26/27Dia 48mm
A19E26/27Dia 60mm
A21E26/27Dia 67mm
A25E26/27Dia 79mm
B10E12/14/17Dia 32mm
BR20E26/27Dia 63mm
BR30E26/27Dia 95mm
BR40E26/27Dia 127mm
C7E12/14/17Dia 22mm
C9E12/14/17Dia 29mm
C15E12/14/17/26/27Dia 48mm
CA10E12/14/17Dia 32mm
G11E12/14/17/26/27Dia. 35mm.
G14E12/14/17/26/27Dia. 44mm.
G16E14/17/26/27Dia. 51mm.
G16.5E14/17/26/27Dia. 52mm.
G25E26/27Dia. 79mm.
G30E26/27Dia. 95mm.
G50E14/17/26/27Dia. 50mm.
G60E14/17/26/27Dia. 60mm.
G80E26/27Dia. 80mm.
MR11GX5.3, GZ4, GU10Dia. 35mm.
MR16GX5.3, GZ4, GU10Dia. 51mm.
PAR16E26/27Dia. 51mm.
PAR20E26/27/39/40Dia 63mm
PAR30E26/27Dia 95mm
PAR36E26/27, G53Dia 114mm
PAR38E26/27/39/40Dia 121mm
PAR40E39/40Dia 127mm
T6E12/14/17/26/27, G13Dia 19mm
T7E12/14/17/26/27, G13Dia 22mm
T8E12/14/17/26/27, G13Dia 25mm
T10E12/14/17/26/27, G13Dia 32mm
T14E12/14/17/26/27, G13Dia 44mm
T22E12/14/17/26/27, G13Dia 70mm