There is often а confusion between these concepts: pocket types are confused with their varieties. E.g. a classic-shirt-pocket and a denim-ticket-pocket are of the same type, i.e. patch pocket...
There is a lot of information on the Internet, but it is not always accurate.
To make it easier for you to navigate, we give the classical definitions of the main types of pockets.
Patch Pocket
A pocket formed by sewing a piece of shaped material to a larger part of s garment or another item.
Slit Pocket
Generally, this is a pocket formed on the underside of a garment, reached through a slit opening. The opening could be worked out as a welt, jetted, zipped, etc. depending on the design.
Side Pocket
A pocket built on a cut-off side piece of a garment, e.g. trousers or jeans side-pockets.
Seam/ Inseam Pocket
A pocket built on a seam edge of a garment, e.g. side seam or any other. They're fast and easy to sew, and almost invisible.
See a convenient table of main forms of patch pockets in our Fashion Vocabulary.