Impressions - The number of times an advertiser's banner or link appears in a visitor's browser.
Insertion Order (IO) - This term is used when we talk about placement of banner or some other advertisment. It is very important in associate, affiliate, CPC, CPM and other programs.
IP (Internet Protocol) - IP specifies the format of datagrams, and the addressing scheme. Most networks combine IP with a high-level protocol called Transport Control Protocol (TCP), which establishes a virtual connection between a destination and a source. IP allows you to address a package and drop it in the system, but there's no direct link between you and the recipient.