A comprehensive explainer on hash, rosin and extracts
Cannabis is covered in little hair-like structures called trichomes. Lots of plants have these, including roses, nettles, strawberries. Some of these trichomes on cannabis have glands within them that produce a type of resin. These are the things that appear like crystals all over your bud, and that you catch in your crystal-catcher in your grinder.
Almost all of the cannabinoids and flavourful compounds like terpenes that you want are produced in that resin, rather than the vegetable material of the plant.
The top of those resinous trichomes is like a ball, a thin waxy membrane containing the resin within. The ball-like top of the trichome (sometimes called the resin head) ,when mature, can easily detach from the rest of the trichome.
Hash is an attempt to isolate these resin heads using sieving. This can be done dry (like the crystal-catcher in your grinder or in the mountains of Morocco and Afghanistan) and is called drysift or in an ice bath which makes what we call bubble hash or water hash. The principle is the same - knocking off those mature resin heads, filtering them through sieves/screens. The benefit of using the ice bath method is you can use fresh flower, which retains more of the flavourful compounds like terpenes present in the fresh plant, and it's generally easier to isolate the trichome heads due to the water.
Freshly collected resin heads look like sand. When dried, heated and pressed together, they oxidise, turn brown, and this is the form of hash you're probably most used to seeing.
Like any powdery (unpressed) or dough-like (pressed) substance, hash can be contaminated to varying degrees, by dirt or plant material or dust or whatever.
Even if you have absolutely pure hash, that is 100% trichome resin heads, there are still many variables that affect the quality.
If the wax membrane of the trichome head is too thick, the hash will not melt well and will be harsher and less flavourful. The smaller the trichome head, the more wax membrane, so plants with smaller trichome heads are generally less favourable. The point being, even 100% hash is not all equal.
Static hash refers to drysift hash that has been sieved, but then refined further using static electricity to remove contaminants.
With hash, there is enormous variability. For example, you can imagine that water hash made in a clean lab and freeze dried will be cleaner than drysift made in a shack on a dusty mountainside. That said, drysift can be better than waterhash, a static-cleaned drysift might not be as good as another drysift that wasn't static-cleaned, all of this. It's all highly dependent on the quality of the starting flower, the hashmaking process, the storage thereafter.
Any remotely good hash will melt when heated. You hold a lighter to any resinous hash and you'll see it melting. THIS DOES NOT MEAN FULL MELT. Fullmelt hash refers to hash that is so good, melts so cleanly, it is as good as rosin. Full melt is when the hash is so well grown and so well extracted that there's no point trying to refine it further.
Rosin refers to a crude extract made by using heat and pressure to literally squeeze the resin out.
'Flower rosin' is where you put a bud of flower in between two hot plates, squish it with lots of pressure, and it squeezes out the resin.
'Hash rosin' is the same process but using the already isolated trichome heads (we wrap them in mesh - imagine squeezing a mesh bag of oranges between two hot plates).
Hash rosin, for virture of hash already being a concentrate, is a cleaner more refined product.
Butane Honey Oil or Hash Oil (BHO) and other solvent extracts
You can use solvents like butane, hexane, CO2, alcohols etc to make cannabis extracts too. These are called solvent extracts while hash and rosin are referred to as solventless extracts, to differentiate.
Butane (BHO) is the most common for making dabs, and it produces a product similar in consistency to rosin. 'Live Resin' is BHO.
Distillate I believe is often made with CO2 but someone can correct me.
Dirtier, more 'full spectrum' extracts that are good to take orally (if you've ever heard of the famous Rick Simpson's oil or Full Extract Cannabis Oil) are usually made by soaking the material in alcohol.
Consuming hash and concentrates Of course you can smoke hash like you can flower, or use your concentrate-pad in your vaporisers. Good hash, rosin and BHO are commonly dabbed though. Dabbing is just a crude form of a vaporisation, where a surface is heated and a small glob of extract is put onto the hot surface while you inhale. You can use a traditional 'dab rig' which is like a bong, or a modern electric device by brands like Puffco or Carta.
Conclusion If you're only interested in THC, you might question why hash rosin is worth more than flower rosin or live resin, when they have similar potency in terms of THC. The same with the difference between lots of different types of hash.
The answer is the enjoyability of the product and also the craftsmanship put into it. Flower rosin can be as potent as hash rosin, but is generally harsher and less flavourful. A world-class full melt hash can be better - more flavour, more potent and more smooth - than rosin. Solventless extracts generally demand more finesse and craft than solvent extracts.
Basically - there are levels to this shit. Not that dissimilar to how some flower is clearly much more enjoyable than others, cos it's been grown, dried and stored better. Hash is similar, but with the added variability of the hashmaking and there are many different ways to make hash. All of these things have their place. Some cannabis, like hazes for example, don't make good hash, so they're better to extract with solvents. Some products are meant to be dabbed or vaporised, some to be smoked, some to be used orally or rectally
It's a hard product to get your head around if you're new, because there is so much variety and different variables to consider.