A Search Engine is the process of using a keyword or specific phrase to search the entire internet by using the sites Search Engine Database (Google, Yahoo, etc).
If the website is not indexed within the database of a particular Search Engine, it may not come up in a search though... even if the user enters the URL perfectly. Another thing to understand is that if a website has not been submitted to Search Engines, optimizing that website may be ineffective.
One way to define Affiliate marketing is the method of promoting an online business by using an an affiliate (a middleman) who is compensated for every visitor, subscriber, customer, and/or sale provided through his/her efforts.
Affiliate marketing is also defined as the industry where companies and individuals are performing affiliate advertising and marketing, including affiliate networks, management and offsite advertising.
This can cross over into other internet marketing methods, because affiliates are using the same methods as a lot of the merchants. These methods include search engine optimization, paid search engine marketing, email marketing and online advertising.
