A generalist species is able to survive in a wide variety of environmental conditions and can make use of lots of different resources. Generalist species have a big niche. Humans, rabbits, house flies, and raccoons would all fall under the generalist species list.
A specialist species can only thrive is a small variety of environmental conditions and/or only has a limited diet. Specialist species have a small niche. Pandas, koalas, and salamanders would all fall under the specialist species list.