In the nice days of the past of film photography the only method to accomplish many effects on your photos was with the usage of specialized glass (or plastic) filters, fitted to leading (or in some cases slotted in to the rear) of camera lenses just before taking the photo. Fortunately in the present day digital world some of these filters continue to be used and while most of the old effects may be reproduced in a digital darkroom, others still require the photographer to do the work prior to publish processing. Starburst Slot
If you speak to the majority of photographers (either professional or amateur) chances are they will probably have some kind of filter in their camera bag, even though they do not use it. While you will find a lot of different types of filter to list them all in more detail, a rundown of the most used will be a good place to start giving advice:
Protective filters: Usually a UV or perhaps a Skylight filter, both often employed to protect the finish element of a contact from slight knocks (it is far cheaper to replace a filter than a lens!). One word of warning though, filters of this type in many cases are bought cheaply, but this seems strange to me, people will pay thousands for expensive camera lenses and then place a low priced piece of glass in front, often affecting the image quality of one's final photo. Starburst Slots
Polarising filters: One of the most used and useful filters, these are used to eliminate or reduce glare from water etc. as well as to give the appearance of more vivid blue in skies, and really are a must for landscape enthusiasts.
Neutral Density filters: These filters reduce steadily the available light reaching the lens, allowing slower speeds to be found in bright conditions (excellent for showing the movement in water), and can be found in several strengths. You can find two forms of ND filter, a complete ND where the whole filter is one dark colour, or graduated ND where the colour effect is started midway through the filter, these used to permit for skies to not be over exposed while exposing correctly for foreground objects.
Special effects filters: The ultimate forms of filter are where you can go really mad, you will find so many different variations to select from, from starburst filters - creating striking bursts of light, to various coloured filters - giving the chosen colour cast to the whole image (Tobacco was once very popular), they are worth investigating in increased detail for a fuller picture, their numbers are vast and different effects produced staggering, they're however, often filters that could easily be replaced with modern software.
Once you have chosen the design of filter you intend to use, you also have to select from different types (and manufacturers). You can find two very distinct forms of photographic filter available;
Circular filters that screw onto leading element of your camera lens, these require many different sizes to match your different lenses, but individually usually are inexpensive.
Square filters these slot into specially made filter holders, the huge bonus with these is that with different adaptors this allows you to play one filter on many lenses, as well as stack them up (generally these square filters are of a greater quality than the circular ones) You might also need the main benefit of being able to alter the starting position of the filter (depending on what area you want the filter to affect). There are lots of specialist manufacturers such as for instance Cokin or Lee (the two most famous and popular manufacturers), in reality way too many to mention.