Topical search engine is an engine that focuses on a particular topic. It covers a part of the whole Web rather than a particular website - this is possible because Google Custom Search allows you to include multiple websites in the same engine. This article discusses some of the interesting techniques for building and maintaining such engines - developers.google.com 
# Why build topical search engines?
It is possible to perform a very precise and robust advanced search using the standard google.com search engine, by choosing the right keywords, utilizing search operators and filtering results. However, some users might not be familiar with these advanced techniques.
Since you know the context your user is operating in and the topic of the search, you can guide the user through the search process and make the discovery of valuable resources in the chosen domain much easier.
# Example
A search engine for students in an engineering course can add engineering keyword to every search, to differentiate engineering results from the generic ones for ambiguous queries like stress.
There are few techniques useful in building high quality Topical CSE's: * Curated sites to search index * Rewriting queries * Exposing additional data in the search results