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The simple definition of endocytosis is 'ingestion by the cell'. It is the process by which extracellular substances enter inside the cell, crossing the cell membrane. Mainly 2 types.

1. Pinocytosis

2. Phagocytosis

Pinocytosis is the entry of substances mainly in liquid/solution form. Actually, pinocytosis itself means 'cell-drinking'.

Phagocytosis is the process by which extracellular substances like bacteria, or any foreign particles are engulfed by a cell. This is the main process occurring in lysosomes, our suicidal bags of cell. And, a very simple example is the case of amoeba. Amoeba procures its food through this process.

 Receptor-mediated endocytosis. In this, the material to be transported, it initially binds to a receptor and then this receptor-substance complex is ingested. Transport of cholesterol and iron takes place this way.