In crochet, as one may guess, decreasing is the act of making fewer stitches than would normally be made in "straight" crochet work. For example:

T\ T\ T\ T\ T\ %
%  T\ T\ T\ T\ T\
T\ T\ T\ T\ T\ %
o  o  o  o  o  o
In the above work, the next row would call for, as those before it, a turning chain and five stitches. However, by using the follow methods of decreasing, one can make fewer stitches in subsequent rows to modify the shape of the piece.

The principle of nearly all decreases is the same, but depending on the stitch being used, the method can vary slightly. Say, for example, that in the above example, you want to decrease from six stitches per row to four. Basically, you would create the five "necessary" stitches (and a turning chain) to fill the places of the previous row, but collapse groups of stitches together so that in the next row there would only be places for 4 stitches.

Single crochet decreases: To begin a single crochet decrease, insert your hook as usual and begin to make your next single crochet. However, instead of finishing by yarning over and working off the two loops on the hook, keep the two loops on the hook. Then, as if you had finished a single crochet, insert your hook into the next stitch to create another single crochet. If you only want to decrease by one stitch (effectively making one stitch where two should go), you can then yarn over and pull all three loops off at once. You'll notice that the single crochet decrease has the width of two stitches, fills the place of two stitches, but only leaves available space for one stitch in the next row.

As you may have guessed, you can decrease by more than one stitch at a time. A double single crochet decrease can be created by by working three stitches before taking off all of the loops. In fact, as many stitches as you want can be decreased at one time, provided there is room on the hook for that many loops. However, as the number of decreases worked in one spot increase, the spot becomes more and more noticeable and eventually becomes lumpy. Therefore, if you want to decrease many stitches in a row, it is best to make decreases of small numbers at regular intervals throughout the row.

Double crochet decreases: These work in the same way as single crochet decreases, but are made slightly differently. The following shorthand is the method for making a double crochet decrease:

yo and ins hk into next st, pull up a lp. yo and work off two lps, then yo and ins hk into next st. pull up a lp, yo and work off two lps, then yo and work off three lps.
A basic rule for decreases of any stitch is this: Work all of the first stitch except for working off the last two loops. Then, make another stitch in the next stitch and work off all but the last two loops of that second stitch. Repeat this for however many stitches you want to decrease, and then to finish the decrease, yarn over and work all of the loops off at once.

Decreases worked along the edge of a work create a smooth, tapered decrease. Many decreases worked over time at the same place in each row create a dart, or eventually, if compensating increases aren't made, a right angle, as in the ripple afghan stitch. If decreases are made scattered throughout rows, the piece will start to slightly curve.

This crochet write up uses crochet abbreviations and crochet symbols.
Source: Crocheting in Plain English by Maggie Righetti