A thinner cymbal will be warmer in sound. Sometimes the company will go with descriptions like med-light, med-heavy, etc. but the old standard way of describing them was:
Viennese-medium weight, all purpose cymbals. If I had one pair of cymbals to buy for band it would be 18" Viennese.
German-thicker, heavier in weight. Pitch is higher, and attack can be brilliant and a little difficult to play with finesse. 17" German are my go to for marches, 18"-20" are appropriate for Mahler.
French-thinner. Very easy to crash, warmer sounding, less harsh in the attack as the plates are more pliable. Good for Debussy, Ravel, crashes for the sake of color rather than impact. I would take a pair of 17" or 18" French cymbals for a church service any day.