Text links are links that you insert into your header or footer. These can appear on individual pages, or every page.
These are attractive to a website owner if you are willing to put different ad text on each page to mix it up a little. Some search engines can identify potential link buying if you have 100,000 pages, and each have the same text in the link.
In cases where the client (website owner) is concerned about this being identified as buying links, then this is the best option for them if you are willing to do the work.
This does requires quite a bit of work but may earn you more in the long run.