Some basic definitions: Stretching

Here are a few more definitions:

We're all familiar with stretching - you attempt to get more comfort and elasticity into your muscles by contorting yourself into position and holding there. There are a few major types of stretching we'll cover today.

Static Stretching involves taking a muscle and holding it in its most lengthened positioned in order to improve it's flexibility or maximum length. This is the most common form of stretching - you pull the muscle into a position until it just starts to hurt (the "pain periphery") and then back down slightly, and hold for 15-45 seconds (sometimes longer, but the recommendation vary). You ease into and out of position. An example of a stretch like this is a hurdler's stretch.

While static stretching before exercise is common, fewer and fewer trainers recommend it. Lengthening a muscle like this weakens the muscle, which makes it less able to shift heavy weights and more vulnerable to injury. While increasing flexibility and getting the relaxing effects of a static stretch are valuable, doing it before you lift or do cardio will have a negative effect. Save it for afterwards - either as a warmdown, or do it before you go to sleep at night. It'll help you relax before bed and you won't need to call on those muscles for a good 7-9 hours or so.

Ballistic Stretching is the opposite of static stretching, and generally isn't recommended. Instead of stretching slowly, you stretching quickly, moving the muscle group up to and beyond the comfortable range of motion. You get into a stretched position and move - usually bouncing - to try and extend the range of motion. This isn't recommended anymore basically because it's potentially injurious (go to far, hurt a muscle, tendon, or ligament). However, some trainers still suggest using these, as they can improve your range of motion and the chance of injury is deemed very low. Use them carefully! An example of ballistic stretching is a hamstring stretch with a bounce.

Dynamic Stretching is more similar to ballistic stretching than static stretching, but seems to avoid the potential for injuries of ballistic stretching and the weakening effect of static stretching. Dynamic stretches are exercises that take your muscle through a range of motion to improve their function and warm them up. These exercises are aimed more at getting you ready for athletic or exercise endeavors than in improving flexibility or relaxing.
An example of a dynamic stretch are glute bridges.

Generally:

Before exercising: Dynamic stretching
After exercising or before bed: Static stretching


More reading:
T-Nation put out an excellent two-part article on stretching. Warning, these contain Not W/FS images. Part 2 contains a complete stretching routine.

Here is an example of a dynamic stretching warmup from Core Performance.

Two excellent (if somewhat expensive) products covering dynamic stretching are the DVDs Inside/Out (Mike Robertson/Bill Hartman) and Magnificent Mobility (Mike Robertson/Eric Cressey). Ignore the hard-sell pages, these are very high quality items.