Occasionally, someone will ask you to link your website to theirs. Typically, you will get an email request from someone saying they will pay you to put a link on your website, or they simply might ask for a link in exchange for linking to you.
Usually, these types of requests are bad for your business and they should be ignored. But, if you entertain the request, you should first make sure their website is not a spam or black hat website.
Here's how you check:
1. Visit their website and review the PageRank value on the Google Toolbar. It needs to be at least a PR1.
2. Read through their website. If you don't like the look of the site or what they are saying, odds are Google does not like them either.
3. Search Google for "site:domain.com" without the quotes. You will get the "Your search did not match any documents." Message if they are blacklisted.
4. Search Google for "link:domain.com" without the quotes. You will get the "Your search did not match any documents." Message if they are blacklisted.
If any of the 4 items above seems bad, you should avoid the link request.
Similarly, before you put a link on your website to a friend's or other business associate's website, you should check the same 4 items as listed above.
You need to be careful who you link your website to. Your linking is an indication that you endorse that other website and Google will hold you accountable, and penalize your ranking, if you link to a spammy, black hat, or otherwise dangerous website.