The acronym ROI stands for return on investment. This business term refers to the measurement of revenue or profit that a company generates from a specific investment or several investments that have been grouped together. Website owners can calculate the ROI on their investments using marketing programs to promote their websites.

Calculating the ROI involves determining the gain from the investment and the cost of the investment. The gain or benefit from the investment less the cost of the investment is divided by the total cost of the investment, resulting in a number that is expressed as a ratio or percentage. This is the ROI (ROI= financial benefit from the investment - cost of the investment/ cost of the investment.) A positive answer or number indicates that the investment has been profitable for the company or business. On the other hand, a negative ROI indicates that the venture has not been profitable.

• wikipedia explanation of ROI
• Investopedia definition of ROI
• SearchCIO.com Definitions
• wikipedia explanation of ROI

AdSense is a program that is run by Google. It is also referred to as Google AdSense. This ad-serving program allows Website owners to make use of advertising images, text ads, and video advertisements on their sites. Administered by Google, the ads generate revenue for the Website owners with a minimum investment in time. Plus, no additional resources are required. The revenue is based upon one of two methods of calculation, per click or per thousand impressions. 

The advertisements are relevant to the content that is maintained on the Website that has signed up to use AdSense. Google’s AdSense automatically delivers ads to your webpages based upon the content that it discovers there. There is no additional effort required on the Website owner’s part. The ads are specifically designed to be relevant to your audience so that they will find them to be useful advertisements rather than annoying bits of information or distracting text or images.

• A Wikipedia explaination of Adsense
• Google Adsense
• A tour by Google of Adsense
• Google's official Adsense blog
• Webmaster World discussion forums on Adsense

POP is an acronym for Post Office Protocol. Mail clients use POP to retrieve their electronic mail (emails) through a mail server. POP3 is the most commonly used mailbox access protocol, partly due to its simplicity to use. Additionally, POP3 can be used by mail clients with or without SMTP or Simple Mail Transfer Protocol. SMTP takes care of transmitting all electronic mail messages or emails between computers, between computers and servers, and from server to server.

POP3 is actually an interface that exists between the mail server and the mail client. POP3 features storage to hold the electronic mail until individual users log in to retrieve and download their personal emails. In fact, every pending message including those with an attachment is automatically downloaded when the user checks their mailbox for incoming messages. An individual email account is often referred to as the person’s POP account.

• A Wikipedia explanation of POP3