Mmm ... at least strictly speaking, hash or hashish is not marijuana (or ganja or grass), although it is a derivative of the Cannabis Indica plant.

The first part of the Webster definition, basically.

Hashish is very common in Europe, mostly because it is so very handy for transportation: it can be packed in bricks, made reasonably waterproof and shipped across the Mediterranean Sea be enterprising Albanians and Italians and Turks and basically anybody that is willing to risk.
Hashish also lasts longer than grass.
Its key defect is that it does not really burn by itself, so making a hashish joint entails:

  1. Toasting the hashish with a lighter: not too much, or it will carbonize and goodbye THC.
  2. Crumbling the hashish together with some cigarette tobacco.
  3. Making the joint.
This is clearly messier than a nice clean spliff or pipe of pure grass, but the price and availability of good marijuana in Europe being what they are, hashish can be your only option.

Hashish is usually known by its real or assumed place of production, so you have Moroccan, Albanian, Afghani ... with different flavors and potency (or so I hear).
One good place to sample different varieties is Amsterdam, where you will easily turn into a hash taster (like a sommellier, but more ... groovy, you dig ?) - supposing that you resist the greener and fresher lure of skunk or Northern Lights.