Long before PhotoShop (for you purists, it's a computer program for digital image manipulation, not to be confused with a camera store), photographers have been using filters. Filters are bits of glass or plastic that are attached in front of, behind, or in the middle of a lens. The most familiar are the type that screws onto the front of a lens. Many people use a UV filter on their lenses, not so much to filter out UV light, but to protect the lens from dirt and damage.
Photography is all about light. One of the characteristics of light is color. Objects don't have color so much as they absorb some colors and reflect others. The color we see is related as much to the color of the light source as to the color of the object. A red ball in a blue light looks black because the blue light source doesn't have any red light for the ball to reflect. Our eyes and brain are pretty good about compensating for different light sources. (The digital camera people call this white balance.) Film, on the other hand, responds to light the same way regardless of the source. Most film is called "daylight" film, which means that it responds most accurately to the color of sunlight. Tungsten film is designed to respond accurately to incandescent lamps. If you shoot tungsten film in daylight, the images will have a blue cast because sunlight has more blue than lamplight. If you shoot daylight film in lamplight your images will be too red because lamplight has more red than sunlight.
One kind of filter is the color correction (or CC) filter. These are used to balance the color of the light with the color response of the film. They are essential and boring and that's all I have to say about them.
Many photographers use a polarizing filter. It works like the sunglasses. Polaroid material blocks light from a particular angle. When light is reflected off water or glass it can be blocked by a polarizing filter set at the proper angle. The polarizing filter is mounted in a slip ring so it can be turned. By looking through the viewfinder and rotating the filter, you can select the filter position that blocks the unwanted reflections. Polarizing filters are also useful for darkening skies. Because polarizing filters are a neutral color, they work as well for color film as they do for black and white film
Black and white photographers use color filters to enhance their images. For example, sky has a lot of blue in it. A red filter would remove blue light and darken the sky. When you look at some of Ansel Adam's images, you notice the very dark skies. He often used a red filter to achieve this effect.
Colored filters don't add color they subtract it. A red filter subtracts blue-green, a green filter subtracts red-blue, and a yellow filter subtracts blue. The effect is to lighten objects that match the filter color and darken objects of the opposite color. A green filter will lighten the color of leaves and darken the color of the red rose. A red filter will lighten the rose and darken the leaves.
Because filters subtract light, you have to increase your exposure to compensate. If your camera has a built in light meter you don't have to do anything, the light meter reads through the lens and consequently through the filter and all is well. With a hand held meter you can compensate for the filter by multiplying the shutter speed by the filter factor or you can hold the filter over the meter's lens when taking the reading.
Except for color correction, colored filters use with color film fall in the category of special effects.
Special effects filters are a lot of fun. There are all kinds, star or cross-screen filters, multiple image filters, masking filters, graduated color filters, defraction filters, center spot filters, and more.
Multiple image filter and ONE rose.
One trick often used by portrait photographers is the soft focus filter. You can make your own soft focus filter easily. Take an old UV filter that's too dirty or scratched for normal use. Smear it with a little Vaseline or bear grease or what ever is handy. You don't need very much. Now shoot that portrait of your wife or mother. A gram of Vaseline will remove more wrinkles than a ton of expensive wrinkle cream.
If you don't have an extra UV filter lying around, you can take an old nylon stocking (that's one leg of a pair of pantyhose to you youngsters) and stretch it over the end of your lens. You don't want anything too heavy-- tube socks won't work. I haven't tried lace stockings, but why not?