Link building is the action of getting other websites to link to yours.

This was the original Google breakthrough: if an important site says you're important, than you are important. IF what the linking site says you are about, agrees with what you say you are about.

Once upon a time - and to some people still - these "inbound links" were of greatest importance in getting search engine rankings.

Now they are often marginal in importance as per Google they only contribute about 20% to your rankings.

And getting GOOD inbound links is a lot of work.

But it is important as a part of your overall Internet Marketing program, if a long-range one. And there are situations where it becomes critical to, say, getting from #4 to #2 or #1 on a key ranking.

To get you started, here are a couple of articles with some pretty good strategies for link building:

Link Building Part 1

Link Building Part 2