All of the below mentioned methods are known collectively as "intensity" techniques. I personally consider them advanced training methods, so if you're just getting into BB'ing/resistance training I wouldn't suggest worrying about intensity techniques. You'll make plenty of progress using just straight sets, adding weight to the bar and eating and resting enough.

I will however give you a brief description of several intensity techniques for future reference.

1) Drop sets- do a set to failure, and then decrease the weight on the bar and continue until you once again reach failure (rest just long enough to strip the weight off the bar, or better yet have a partner or two there to strip the weights for you). This process can be repeated several times (i.e. "double drop-set" "triple drop-set").

This method is good for stressing a wide range of muscle fibers very efficiently. In other words start with a fairly heavy weight (say 5 RM/fast twitch fibers), then when you reach failure drop the weight (now you'll be hitting more intermediate fibers), when you again reach failure drop the weight again (now you should be using fairly light weight and stressing more of the endurance fibers).

2) Pre-exhaustion- perform a set (to failure) of an isolation exercise for a specific muscle and then immediately perform at set of a compound exercise that also works that muscle (example leg extension/squat).

Generally with pre-exhaustion the purpose is to emphasize a specific muscle that you have a hard time recruiting during the compound. So, while I used leg extensions (quad) and squats as an example, that's probably not a very common example (most people tend to be quad dominant squaters naturally). A more common example might be something like straight arm pull-downs and pull-ups (for someone who has a hard time recruiting their lats).

3) Post-exhaustion- Basically the opposite of pre-exhaustion. Perform a set of a compound exercise and then immediately perform a set of an isolation exercise for a specific muscle. This method allows you to use slightly more weight on the compound than pre-exhaust, so some people prefer it.

The idea is to really stress/overload the muscle with the compound and then use the following isolation exercise to really pump it up and get lots of blood/nutrients into the muscle belly.

4) Super-sets- (Note: this is the method that Arnold preferred for super-sets, though some would consider pre-exhaust and post exhaust to be forms of super-sets). Perform an exercise for a muscle group and then immediately perform an exercise for the opposing muscle group (example bench and rows).

This method is a good time saving method as is allows you to do two muscle groups in about the same time it would normally take you to do only one. Some suggest that it also actually improves strength/performance in the lifts as the muscles not only rest while the opposing muscle works, but also provide less resistance to the opposing muscles due to fatigue.

5) Rest-pause- as originally invented by Mike Mentzer (or at least I believe it was) was basically the same thing as what CT describes as "cluster sets" in his newest article. Many people however have come to associate the term with Dante Trudell's methods. Basically Trudell's method is to take a set to failure, take 10-15 deep breaths, go to failure again (keeping the weight the same), and repeat one more time.

This method is very good for building strength (both the original method and Trudell's method) as it basically allows you to get more reps with the same weight in less time than you normally could.

6) One and a half's- is another method designed to emphasize a specific muscle group during a compound exercise. Basically you do exactly as the name implies, perform a full rep followed by a half rep.

For instance during the bench the chest/pecs are primarily responsible for the lower half of the movement and the triceps are primarily responsible for the upper half (or at least that is the theory). So, if you wanted to emphasize your chest you'd do a full rep, then come all the way down to your chest, come up half way, back down to your chest and then all the way up again. If you wanted to focus on the triceps you'd just do the half rep in the upper half instead.

7) Partials- generally partials are done with supramaximal weights (more than you could lift through a full range of motion) and are used primarily for building strength. Once again, check out CT's latest article for an example of a method utilizing partials. CW also did a good article a while back that was completely based around using partials.

8) 21's- are performed by breaking a movement into three ranges/parts (a bottom range, a middle range, and a top range) and then performing 7 reps in each range. It's primarily a "pump" exercise designed to get a lot of blood/nutrients into the muscle.