That can't be right! Shouldn't ten links be better than one? It is usually the case where more is better. However, when it comes to gaining links, having more links isn't necessarily better than having a few good links. In fact, having too many irrelevant links could possibly harm your website's visibility within the search engines.
These days, search engines are placing more emphasis on incoming links to a website when evaluating rank. The reason being is because they are difficult to obtain from trusted sources (a "by the book" website such as the Dmoz directory) and it is a good way to partially eliminate the group of people who abuse the "on the page" factors of optimisation (page spamming).
How is this so? Well, if you have someone who is abusing the "on the page" factors and they have no trusted links to their page (they will most likely not get any at all due to spam) then there is no chance of them getting a good listing. However, people have worked out ways in getting a large number of links, through link farms and other tactics, regardless of the quality of their website. As the search engines are aware of this, it brings us to the topic of how one link can be better than ten.
Obtaining one link from a trusted source can be very difficult in the sense that they do not issue links to just about anyone. If you have a good website with relevant information, then you will get them. It is for this reason one link from a trusted source can better a multiple of links from other websites. As a trusted source website is "trusted" by search engines, it will also trust the websites it links to.
If you have ten links from your best friends, it will help you in some sense, but no where near as much as the one link from a source like Dmoz, unless one of your friends has a very reputable website amongst the search engines.
Having too many irrelevant links could harm your website's visibility in the search engines. What is meant by this is having links from websites that have no relation to yours. An example of this would be a site about cars linking to another site about medicine (unless the sites are reputable and have a good reason for the link i.e. there is a relationship made between cars and medicine).
Search engines will pick up on this sort of link and will give it no weight in boosting your visibility. Obtain many of these irrelevant links, then it could be seen as spam (like a link farm tactic) and that will lower the popularity of your website within search engines.
The key here is to focus on quality and not quantity. Gaining a few good / relevant links will take you much further than a lot of irrelevant links. To get these quality links, start off by placing your website in directories, but be sure to avoid any "on the page" spamming as this will not get you accepted.
Once search engines have acknowledged a presence of relevant links, follow up on it and research some ways to obtain more. As time passes, you will notice that your website's performance will increase, as the links age, provided you have carried out search engine optimisation. Avoid obtaining the irrelevant links even though it may be tempting to see fast results. It will only be a matter of time before a search engine realises the spam tactic, and as the saying goes "fast rise, fast fall".