Mexican ceviche vs. Japanese sashimi
Even with the same fish, sashimi and ceviche are very different dishes:

Sashimi is sliced, raw fish that is in its natural state.  Note the glossy, translucent nature of the slices:



Sashimi is also just the fish, although the plate may have additional garnishes.

Ceviche is technically uncooked, but has been marinated in citrus juice which coagulates proteins and turns the flesh opaque in a manner similar to cooking.  Ceviche is often combined with onions or other complimentary vegetables.

Note the opaque, white color of the flesh:



In terms of the fish used, sashimi is often made with oily fish like tuna, yellowtail, or snapper; ceviche is more often made from white fish, or even shrimp or squid.  However, there is considerable variation in both cases.

See also:  Food Lab article on ceviche