A render is the process of generating an image, wether it is a photorealistic one or not, from a 3D or 2D model using a computer software. It can generate a group of images from each frame of an animation to create a video and moving image. It is a process that requires a long time to perform and good equipment, it is the way to produce a final image.
A playblast is something similiar but does not have the benefict of the good quality, it is maily used to preview the work that is being done in the animation software, but it is much faster at making the video, so it is a very usefull tool for animators to preview their work before rendering it.
So, both have qualities and flaws, and each is used for different porpuses even if they seem the same. Render is for a final result and good looking images(frames) and playbast is to preview the work while doing it and before achieving a final thing.