Accumulating links is a vital component to your search engine ranking, and some SEO companies obtain links by buying them. All links are not created the same, some links from a high PR website are worth hundreds more than links from low PR sites.
If your SEO expert has made your search engine ranking rise very rapidly without content he/she is very likely to have achieved it by spamming methods like link farms or buying junk profiles.
A link farm is where a group of web sites that have been set up for the sole reason of creating links between so that many links are created. This type of technique is often risky, and generally requires you to continue with their service because as soon as you stop, your rankings will drop. In addition, you stand the risk that a link farm could be detected and all those who participate will be delisted from the search engines.
Profiles, are where an account is created on a website for the sole purpose of obtaining a link. Profile links are often deleted by moderators who are looking for this type of abuse of their website. So if you use someone, who uses junk profiles to obtain back links, you have to buy lots of them and often continuously to maintain your rankings.