Following are the five main guidelines for links:
Make obvious what’s clickable: for text links, use colored, underlined text (and don’t underline non-link text).
Differentiate visited and unvisited links.
Explain what users will find at the other end of the link, and include some of the key information-carrying terms in the anchor text itself to enhance scannability and search engine optimization (SEO). Don’t use “click here” or other-non-descriptive link text.
Avoid JavaScript or other fancy techniques that break standard interaction techniques for dealing with links.
In particular, don’t open pages in new windows (except for PDF files and such).
Links are the web’s number one interaction element. Violating common expectations for how links work is a sure way to confuse and delay users, and might prevent them from being able to use your site.