When it comes to strength there are many different kinds of it. Strength is an important factor for overall fitness. It is also needed to develop abilities such as power, speed, and stability. It also helps agility, mobility, and muscular endurance.
Absolute strength is the absolute amount of force that can be put out by a certain muscle. It would be hard to have an actual number for this since it's more of an estimate then anything else. The reason that it's more of an estimate is because to get an absolute strength number, you will actually have to disregard all the safety controls over your muscles to get to it. Most of time the only see absolute strength in emergency situations.
The next kind of strength is maximum strength. An example of this would be the amount of weight that you can lift one time. While it is nice to know these numbers most of the time maximum strength is rarely important for training. You can usually figure out a person's maximum strength by a six rep maximum test.
Relative strength is the next way to measure strength. It is the maximum force output divided by an individual's mass. For example, a heavyweight wrestler might be able to physically lift heavier weights than a lighter wrestler. However, when their weight is taken into account the lighter wrestler might have a higher relative strength ratio than that of the heavyweight wrestler.
Explosive strength is next. It is measured by the maximum amount of force that can be put out in the shortest amount of time.
Endurance strength is strength needed over a certain amount of time. A good example of this would be a bike rider pedaling up a mountain.
There are many different ways for you to increase your strength. The most common way is to lift weights. When you lift weights you are for the most part, increasing your maximum strength. There are certain weight lifting exercises that you can do to build up your explosive strength though. An example of this would be a clean and jerk. If you want endurance strength then your best bet is to lift but with lighter weights and many repetitions.
It is important that you don't focus on just one type of strength building activity. You will get the best results when you combine the different kinds of strength such as maximum strength and explosive strength. Overlooking one or the other can hurt you in the long run.