The art of hacking is a complex area, and many years are needed before a hacker starts to master them. There are two main topics on which he can reach power. One is the pure, brute way, the other is the fine way of an expert. Both are equally important and it does not pay off to only concentrate on either PL or System Knowledge.

Powerlevel

The powerlevel of a hacker describes his raw force. It shows how well he can handle his gear. It also indicates how well he can adapt to any given system. So the PL as a whole indicates how well a hacker can adapt to a given situation and thus how many action points he can gain out of it.
The world is not so simple that one either knows all or knows nothing about various system details however. Instead the PL of a hacker is a numeric value. A 1 indicates only very basic skills while an 8 or even more is the sign for almost infinite power. The PL is not limited, but at a certain point increasing the PL even more does not make much difference.

System Knowledge

The second way to improve one's power. Instead of the raw force one can aim to learn how to hack a system more efficiently and how to save action points on procedures, or he can learn how to pick from a wider variety of systems and so always have a better adaption.
Systems define not so much what you do, but how you do it. There are many ways to skin a cat, and the act itself does not say much about your style.

There are 10 known schools of hacking:

Standard

As the name implies this is the most widely used one. Almost everyone starts his work with it. It is a solid collection of various procedures. There is almost nothing that you cannot do within this operating system. On the downside the lack of specitivity causes a certain unspectacularity. Don't expect anything fancy from this school. Also standard tends to be quite unstable. On the library side standard libs tend to be a bit on the smaller side compared to other schools. This makes them ideal for fast strikes or as basis for bots.

Turtle

A turtle doesn't attack. It relies on its thick shell. Unsurprisingly the Turtle method does the same. Hackers relying on Turtle sacrifice attack power and intrusion power for sheer shielding. This property makes the Turtle guys the master of defense and reconnection of sockets. On the downside libraries that build on this property are kinda hard to use. They can only hope that the system itself disconnects the enemy before it disconnects you. It is strongly recommended to not only rely on Turtletech.

Trashbin

Trashbin was one of the greatest hackers of all times. Sadly he died in the great collapse. His knowledge however remains. Systems working on his principles are known for their high adaptability. On the weak side stuff derived from Trashbin's work are hard to modify and quite costly (can you say patent law? ;) )
Still, if you got the money Trashbin is able to do almost everything that Standard does, only better.

Venom

Venom means patience. In a certain way Venom type stuff is rater similar to Turtle. Where Turtle relies on easy reconnection and high defenses Venom often uses heavy built in error correction. As the result the procedures of Venom are more geared on softening up the enemy then to protect oneself, as defense is already covered in its excellent libs...

Ninja

The Ninja brand systems are the masters of shadow. Not especially strong or reliable, Ninja works with different concepts. A Ninja library is almost invisible, making it extremely hard to hit. On the other side it is usually a master in intrusion as well. A deadly combination. The procedures are also designed to further enhance this effect. If one prefers fast operations instead of long wars Ninja is definitely a must buy.

MegaHard

Turtle, just worse. In its long tradition every piece of commodity that came out of that corporation was more bulky then its predecessor. That doesn't mean its necessarily bad, but most hackers still prefer to avoid it. A pretty steep price structure also helps to build that aversion.
All in all MegaHard stuff tends to be easily hit, but it is well known for its ability to take that hits.

CUR53S

To name CUR53S a system is a very far stretch. What hackers call CUR53S is more like an unmaintened heap of alpha grade software. There are some real marbles in here, but most stuff is weired. CUR53S stuff also tends to need a fair ammound of processor power just to keep the mess together. If having in depth knowledge of the target system CUR53S based libraries are often the most efficient ones however. Also CUR53S based procedures are well known for their mass targetting properties.

BullsEye

BullsEye is pretty similar to CUR53S, it is a bit more refined however. In fact the schools share about 70% of their code. Unfortunately the remaining 30% are rather different, preventing the knownledge transfer from one to the other. Where CUR53S sets on strength BullsEye builds on tricks. On the procedure side BullsEye has the nickname sharpshooter. While weaker then CUR53S the procedures are far superior regarding targetting. Some even menage to hit the same enemy 4 times in a row!

Classified

It is hard to say anything about this system. It isn't actually just one system. Most classified systems work quite differently from each other. As such they are as versatile as the standard one if not more. Just that they are rather - chaotic. Classified Systems are formed by a bulk of hacks put together. And that is what you get. Classified procedures are probably the strongest ones available, only second to Military stuff. But you can never be sure what you get. A classified procedure might always have some unknown sideeffects. It migth even harm its user. It is strongly recomended to stay away from this stuff.

Military

No information is available on Military Systems. Obviously they are the best of the best - just too expensive. Military software works exceptionally well against Turtle and Ninja systems, but also most other systems got big problems. Astoundingly the Standard software, as well as then stuff from MegaHard, have rather few problems with Military gear.

Again the world is not so simple that one either knows or knows nothing about a certain system. Instead, how much a hacker knows about a specific one is approximated with four levels of knowledge:

Nothing

Well, that is obviously something to avoid. If you don't know anything then you are out of the community...

Basic Knowledge

The first level of training, basic knowledge provides all the details one needs to use most procedures from this system. It does not yet refine that use or teach details, which means much power gets unnecessarily burned to achieve the desired results.

Advanced Training

In-depth study and extensive training provide the additional rafinesse that makes the art easier to handle and much more reliable. This is the level most hackers aim at. With the exception of some very rare and complicated procedures, advanced training opens the whole arsenal of stuff the school has to offer.

Mastery

Not many people reach mastery. This is mainly because mastery requires extensive dedication and much experience. When the hardships pay off, however, hacking becomes easier, almost natural. In addition, the long training turned it almost into second nature to the hacker, whom has a much easier time handling the more intricate aspects of the art and can run even the most esoteric procedures.