Personal Information Manager.
A PIM is a computer program designed to manage people's notes: addresses TODO lists, calendar information, etc.
Examples: PDAs are very often (only) used as a PIM. Some examples of PIM software: Outlook, Lotus Notes, Microsoft Address Book.
Traditionally, PIM software have a strict structure on the data - almost like a relational database.
However, there is a whole class of PIM software systems using a much more relaxed structure, i.e. free form databases. The simplest example is notepad - the user creates his own structure - nothing is enforced. Other more complex software helps the user structuring the data, e.g. outliners. A key example of an outliner is TreePad, organizing your notes in a tree structure.