In the process of pinocytosis the plasma membrane froms an invagination.

What ever substance is found within the area of invagination is brought into the cell.

In general this material will be dissolved in water and thus this process is also refered to as "cellular drinking" to indicate that liquids and material dissolved in liquids are ingested by the cell.

This is opposed to the ingestion of large particulate material like bacteria or other cells or cell debris.