Engine: Software that performs a very specific and repetitive function in contrast to an application that has many functions offered to the user. For example, a "search engine" or "database engine" responds to user queries over and over again.
Search Engine: A computer program that electronically searches the contents of a database to locate specific information.
Search Engine Optimization (SEO): methods used to achieve favorable placement in search engines results page by making the pages of a web sites more desirable.
Pay Per Click (PPC): An auction based style of advertisement were the highest ranking goes to the highest bidder and the advertiser pays according to how many times an ad is clicked on by users.
Search Index: A search index is the repository of all of the documents that a search engine has available to search.