is a link to your own site in the context of a useful answer to another posters question. That answer should be more that "look at my site", but be relevant and informational on its own.
For example: a link to your site would be acceptable to me in the context of an answer to a question about finding the right web hosting service for some particular purpose. You could write a few helpful paragraphs, directed at the needs as formulated by the user, or explain a few headlines for relevant factors if his requirements aren't clear. And then you might add a link to your own (useful indeed) website for a more in-depth coverage.
It would be considered spamming if you did it to every such question, of course.
As Bob said, we do have the opportunity to advertise on these forums. But you have to pay for that, as you will certainly understand.
Hope this helps.
Kees