There are several different steroids with slightly different effects on the body. A true regimen of steroid usage includes several different types as well as a recovery drug. Most often a big anabolic steroid is taken at first. This causes your muscle cells to suck in and retain water. This does several important things. One, it stretches the muscle cells. Two, since the cells are swollen, they are stronger for being pushed at their max. Three, with the added water in the cells the protein chains within the cells break down easier and faster. Four, the water helps prevent cell damage while working out.
It also has several bad side effects if used improperly. Too much water in cells for too long will cause decay and can over stretch cells to damage them. The anabolic steroids are typically highly toxic to the body and can cause major liver damage.
This is why most people on a steroid regimen go through cycles. They take an anabolic for a week or two and work out like mad, all the while drinking as much water and sweating it out as fast as possible. They also take in tons of protein as well. People during this period of the cycle have a "moon" face look to them as their faces get swollen as well from the added water retention.
The next part of the cycle is to switch to a steroid that helps build protein chains and allows the water to drain out. The now stretched muscle cells with the broken chains of protein start to deflate as water drains out when the anabolic steroid is no longer taken. To prevent saggy muscles and other problems associated with the stoppage of the anabolic, a more androgenic type of steroid is taken. Usually something that forces protein chains to rapidly form. If done right, the muscles cells that have recently been stretched and had their old protein chains broken are ready to have them rebuilt bigger and stronger. Lighter, shorter, but faster workouts are done here with tons of protein eaten around the clock. This cycle continues for 2 weeks, and then another 2 on an even more androgenic style steroid.
This typically completes a "6 week" cycle iirc. Some athletes will do a few more 6 week cycles to repeat the process a few times to make sure they got the most stretched out and longest protein chains their muscles can hold.
The problem is that no one's body is designed to hold onto that much protein in their cells. Without constant working out and some regular steroid usage, the body's natural chemical called myostatin will start to break down larger than normal protein chains in muscle cells.
Anyhow, one of the other side effects of steroids is that all steroids are basically testosterone. The body reacts differently to different type of testosterone, but one universal effect is this. Your body will stop producing any form of natural testosterone the moment the receptors for those hormones are maxed. In fact, the male body will start creating a chemical that will "aromatase" the extra testosterone floating around in the blood stream. What that basically means is that it will change testosterone into estrogen. The hormone receptors for testosterone also readily bond with estrogen as well. This is a big problem with steroid usage. The other universal effect is that when testosterone production stops, it doesn't start back up for several days, to a week, or longer sometimes. Since the body started converting extra testosterone to estrogen and doesn't automatically start production of testosterone, there is going to be way too much estrogen floating around the blood stream. This will completely undo everything an athlete tried to gain during a steroid cycle. Protein chains will break down, fast will grow, and testicles will shrink, and other problems occur.
This is why people then take Clomid. It was originally designed as a fertility drug for women. It basically binds to estrogen receptors males to stop them from using estrogen. The other effect of Clomid is it tells your body that you have zero hormones floating around in your system, either estrogen or testosterone. This forces your body to start production of it's main sexual hormone. For women it's estrogen, and for men it's testosterone. This is why it's a fertility drug. The extra estrogen for women helps them drop eggs and helps with other parts of the female body to get pregnant.
After all that, many athletes will take yet one more steroid called Winstrol. This one is really different that other steroids in that it doesn't cause the same problems as other steroids such as stopping production of testosterone. It is actually rated safe for kids and women and is one of the few approved steroids they can take.
The main affects of Winstrol are to tell your body to burn nearly all it's fast by hyping up your metabolism to high levels. Also it forces any remaining water in the muscle cells to drain out completely. Most body builders before the actual show take this. It is metabolized very fast in the body and thus is harder to catch from drug testing. It gives people a massively "ripped" look as well as increasing reaction times of muscles and ligaments by removing the excess water that might slow them down. It also has some other side affects that deal with increasing your brains connectivity performance. Which means you can think faster and react faster to an extent. Your perception of your surroundings go up big time.
Anyhow, steroid usage can have a major impact on a persons athletic performance, which is why people want to take them if they can get away with it. And yes, I do attribute the huge droppage of steroid usage as a big correlation to increased "pitching" stats as well as decreased hitting performance for baseball. There are other factors for sure, but this one is a big one in my opinion.