Photography gels are precisely colored acetate sheets (overhead transparencies) used to alter the color of lights. There are different reasons for doing this, including for aesthetic effects and contrast reasons. Professional sets can cost a fortune, these cost me $14.25 (not including the transparencies, which I already had), and are great for any amateur photographer!

Step 1: Making Them

I went online and found the standard color values for a set of gels. Using these, I made an 8x10in jpg file for each color. Attached is a zipped file containing all 27 images that you can burn onto a CD.

Once this is done, take the CD and a pack of acetate sheets to Kinkos, or a similar printing venue. Kinkos only charged me $0.49 a print.

Easy as that.

Gels.zip
Download

Step 2: Using the Gels

Once you've made the gels, it's time to have some fun! There are different ways to use them; for a studio strobe, you can get a filter holder that will attach onto your light stand. You can also get filter holders for normal speed lights. For an on camera flash, just cut out a small rectangle and tape it right on. Because they're made out of acetate, the tape should come right off, making your gels removable and re attachable.

Store gels in a manila envelope to keep them from getting dusty or scratched.
If you're shooting in black and white, a red gel will add more contrast to the picture.
An interesting effect can be achieved by lighting different parts of a picture with different colors.
Putting a gelled light between you subject and a white backdrop allows you to have whatever color background you want.
Position the subject far from the backdrop if you want colored light on the subject, but not the background.

The pictures are just 2 snapshots so that you can get a feel for what effect they have. Both shots were taken with an on-camera flash, supplemented by a lot of natural light. (That's why they are so lightly colored)