You wanna read
http://www.bluelight.ru/vb/showthread.php?t=172649.
PCT stands for post cycle therapy and is performed after the completion of a cycle of steroids/hormones.
First off a basic lesson about hormones. Sex hormones (and steroids) can convert to other hormones and steroids via enzymes in the body.
The 5-AR enzyme converts testosterone to DHT (dihydrotestosterone). This is a more androgenic hormone (androgenic hormones produce secondary masculine effects- muscle hardining, low body fat, deep voice, extra body hair, high libido. They also can agrivate genetic disorders/conditions such as balding, prostate enlargment/cancer and spots). Several steroids can be turned in to a more androgenic dihydro form after coming into contact with this enzyme. 5-AR inhibitors such as finasteride block this enzyme- helping to reduce unwanted androgenic side effects. It is also worth noting that androgenic hormones help reduce the impact of aromatase (see below).
The aromatase enzyme converts testosterone (and other sex hormones/steroids) to estrogen (or an estrogenic form of the hormone). You should know about estrogen- it produces feminine characteristics such as fat storage especially around the hips, thighs and breasts.
An aromatse inhibitor blocks this enzyme- reducing the amount of estrogen that can be produced by your body to practically zero within a few days.
A SERM (selective estrogen receptor moderator - or is it modulator, whatever...) binds to estrogen receptors. This stops estrogen binding to receptors and producing the estrogenic effects. This can almost instantly stop estrogenic effects by hogging all the receptor sites. Its similar to the way SSRI's work if you know about them...
HCG is a hormone usually produced by women when they become pregnant. This looks similar to LH (Lutinizing Hormone). LH is one signal that the body uses to start testosterone production in the testes. HCG is not used during PCT as it limits natural LH production- slowing recovery time.
During a cycle you have a large ammount of testosterone in your system. This can produce a larger amount of estrogen than usual through interaction with the aromatse enzyme. Once the cycle is stopped the testosterone that you have been using is all uptaken by the androgen receptors (this uptakes testosterone, DHT and most anabolic steroids). Now all you have left in your system is estrogen. Basically you have the hormone profile of a 16 year old girl. And your body will make you grow some nice tittys.
By using a combination of the above products you can stop this happening. Using small doses of an aromatse inhibitor (AI for short) during a cycle can stop the testosterone from converting to estrogen in the first place. This isn't 100% positive though as estrogen has some beneficial effects for bodybuilding. It is required for IGF production, it lubricates joints, it helps maintain body water (which will help you lift more) and its absence is usually accompanied by a loss in libido. So go light.
Using HCG in the last 2 weeks of a cycle (or a small dose weekly during the cycle) can aid with restarting your natural test production once steroid use has ceased. Having no test in your system will result in you losing all those gains that you made during the cycle.
A SERM during PCT I think is esential. Any estrogen swirling in your system will not bind to receptors. It also helps agonise gondrapins- which basically means amplifies all the signals that your body is releasing telling it to produce more testosterone to restore natural balance.
This is THE MINIMUM that you NEED to know before undertaking a cycle. I would suggest doing A LOT more reading before undertaking a second cycle. Fortunatly the body can be quite effective at sorting itself out- which is why you may be ok after your last cycle, but its not worth risking it...
Oh and a final point steroids shouldn't be used by anyone under 21. Your body is still developing. Steroids can stunt your growth and reduce natural hormone levels. I dunno how old you are- but thought I'd just toss that in there anyways.