Tretinoin is a prescription strength retinol product. It's basically a very strong version of retinol, which is why you can't just buy in over the counter, as it can be quite irritating when you first use it.
The reason it's so good, is that it has decades of clinical research proving its efficacy in reducing and reversing the signs of sun damage in skin. It does this by increasing cellular turnover (which slows down as we age) and also over time increases collagen production in the skin (which also decreases as we age). Basically makes your skin act like younger skin.
Unfortunately, like I said it can be irritating to the skin, causing peeling and redness for the first few weeks when you use it until your skin acclimatises. Which is why retinol products were developed, they are weaker, but also much less irritating. You can buy retinol in lots of different products, and it will technically do the same thing, but it won't have as dramatic a result as tretinoin and may take a lot longer to see results.
If you want to get your hands on some tretinoin you can either
a) go to a dermatologist who can prescribe it,
b) sign up to a subscription service, if you are in the UK (Dermatica or Skin and Me, you have to pay monthly and they post it out to you), or
c) like me if you aren't in the UK, you can try getting it online from an overseas pharmacy. I get mine from Farmacia Andorra and the brand is called Retirides. If I lived in the UK I would use a subscription service as it's much less hassle.
If you are going to try it, just beware it is a long road, so be patient, it's not a quick fix. It may take a few months to see the best results, and the first few weeks your skin may look worse. For that reason some people prefer to stick to over the counter retinol, which can still give good results.
Also, make sure if you are going to use it that you wear sunscreen everyday, rain or shine. As you need to protect your skin from further damage.
Overall it's a bit of a faff, and an investment of time, money, effort and patience, but it's definitely worth it when you see the results. Which is why people rave about it.
Oops, apologies for the essay!