Some of my research as a biologist concerned psychoactive plants, so in some ways I'm an expert (!), but the street scene is moving so fast that in other ways I have no idea what's actually happening now.
Weed refers to the unrefined plant itself. The female buds have the highest concentration of active ingredient. Smoking or eating that is analogous to grinding up coffee beans and drinking coffee. (Eating it avoids the issues associated with smoking, but it's much harder to control dosage since the effect takes longer to be felt.)
There are two main ingredients: THC which produces a high, and various cannabidiols, CBD, which do not produce a high but are actually more useful for pain relief.
THC can now be synthesized. Some of this lab-made THC is of dubious quality, unknown strength, and unknown purity. It's a thing now to add it to weed sometimes to increase the kick. Plus all kinds of other compounds are also sometimes added, on the more has to be better principle.
So, yes, sometimes weed now is a whole different thing than what it used to be.
The THC is concentrated in the resin produced by the female flowers. Hashish was the historical way to concentrate that resin into a solid block. Now chemical solvents can be used for the same purpose. Depending how careful the producer is, there may or may not be more traces of solvents left in the product than you would like.
Taking in pure THC, to continue the coffee analogy, would be like taking pure alkaloid caffeine. In the case of caffeine, you'd probably wind up in the emergency room. In people whose biology makes them susceptible increasingly high concentrations of THC does lead to increased likelihood of psychotic episodes, which is (duh!) very dangerous. For some people, the THC content of hashish is enough to trigger problems. For most people, plain weed does not.
So, anyway, summary: plant (low THC content) -> resin (medium THC) -> chemically refined or synthesized THC (very high content).