What is Endocytosis?

Endocytosis takes place where the cell recives matter, such as nutrients and ions by the process of invagination of its membrane.  
The matter is taken in and forms what is called an endosome. There are three types of endocytosis and each type will involve the formation of an endosome. 
The endosome will later form an early endosome, where all the endosomes brought in will combine with each other. 
The early endsome created has a number of fates, one of which could be that it is recycled back into the plasmalemma to expand the cell membrane.  
Another in which it could be turned into late endosome as it proceeds towards the centre of the cell where it would then evolve into a lysosome with the help of the golgi body.

The three types of endocytosis are as follows: 

Phagocytosis - which usually involves a bacteria or a virus. Phagocytosis is a process by which certain living cells called phagocytes ingest or engulf other cells or particles. 

Phagocytosis takes place when the cell detects the presence of bacteria and attempts to overtake it. It phagocytosizes it and, by consuming it, will then form an endosome. 

Bulk endocytosis is where the cell needs to take in some water or fluid to expand its volume. When doing this takes in ions and it also forms an endosome.  

Receptor mediated endocytosis takes place when the cells receptor detects cargo molecules on the outside and with the help of adaptins (adaptins help in the specificity of the cargo) the cell can take in the cargo molecule. In this case a protein is detected by the receptor. The membrane then forms what is called a coated pit where it slowly invaginates the substance as well as a receptor and an adaptin, forming an endosome.