POP stands for post office protocol — an Internet standard for sending and receiving e-mail. You can configure most e-mail servers to support it; options include downloading messages from the server and leaving a copy on the server or deleting them from the server. The protocol operates on port 110.
Data is most commonly downloaded to a .PST (personal storage file). Legacy versions of Outlook in particular had difficulty managing .PST files over 2 GB in size at which point the possibility of corruption and data loss increased. More recent versions handle larger .PST files better. The major drawback to using POP is that passwords are transmitted in the clear; however, there is a secure POP protocol that operates on port 995.