A Scientist is a member of staff who works in the player's park in Theme Park World and Theme Park Inc. In Theme Park World, they just wander around, researching new rides, shops, sideshows, and features, and improve existing rides as well in the Research Lab. In Theme Park Inc, they work mostly the same but require a Research Lab building.
The more scientists that you employ, the faster the team of scientists work and improve existing rides and come up with new things for the park. In Theme Park World, after hiring lots of scientists, the hiring menu will not allow you to buy any more - the list of available scientists will permanently be empty, until you sack some of yours. In Theme Park Inc, you are limited by how many Scientists can work at once on a project at one time.
The skill level of Scientists also affect how fast they work. A higher skill level means the scientist works more effectively. Scientists with a high skill level can be hired from the hiring menu, or alternatively, you can increase the rate your scientists increase their skill by increasing their employee training cost in the financial menu. Unfortunately, in Theme Park Inc, the only way to raise a scientist's skill is by making them research in projects.