Puppies have a narrow window in which to experience as much of life as possible, for most dogs this is between 12 and 20 weeks old (depending on breed). Failure to socialise your puppy can result in fearful or aggressive behaviour towards something that is unknown.
Obviously the biggest drawback to early socialisation of your puppy is vaccinations, as your puppy must not be exposed to any source of infection until their vaccinations are complete.
However this doesn't mean we need to lock our puppies away, they can be socialised with people i.e. men, women, children (closely supervised!), teenagers, old people, disabled people and people using walking sticks and wearing hats.
Once their vaccinations are complete we can then socialise them carefully with dogs - as many different breeds as possible (but only where the other dog likes puppies, as all early experiences need to be positive).
Socialisation also includes having experience of:-
Different animals i.e. cats, birds, farm animals (where it is safe to do so from behind a fence) and horses.
Walking on different surfaces i.e. concrete, pavement, real grass & fake, gravel, wood, laminate and tiled.
New environments i.e. shopping centres, vets, elevators, different houses.
Different toys and objects
Noises - traffic including cars, busses, vans, trucks, trains - not only on dry days but wet days too.
Remember when socialising your puppy do it safely, and ensure that they are a positive experience for your puppy.
If your puppy shows any sign of fear to anything that they are introduced to, then remain calm and move away. Never force your puppy to approach something that they are afraid of.
This is a fantastic poster from Sara Reusche and Lili Chin - a great little guide to socialisation.