-Cytosis is a suffix that either refers to certain aspects of cells ie cellular process or phenomenon or sometimes refers to predominance of certain type of cells. It essentially means "of the cell". Sometimes it may be shortened to -osis (necrosis, apoptosis) and may be related to some of the processes ending with -esis (eg diapedesis, or emperipolesis, cytokinesis) or similar suffixes.
There are three main types of cytosis: endocytosis (into the cell), exocytosis (out of the cell), and transcytosis (through the cell, in and out).