If you do a Google search there are a few info-graphics that sum it up.
Both are great tools for different things.
Basically PS is for pixels (photographs and the like) but does SO much more
AI is vectors which are kind of shapes that are non-lossy and can be scaled to any size without losing quality.
Again, do a Google for vectors v rasters.
As scotty says above - vectors (illustrator) work on mathematical points and can be scaled without loss of data whereas bitmap images (photoshop) pixels are fixed and cannot be resized without 'some' loss of data - pen tools work basically the same in both illustrator and photoshop, you use editable 'anchor points' to create a 'path'