Back links are links from other websites to your site - these are highly affective in terms of SEO. The more back links you have the better Google will think your site is. You should focus alot of your time on building the number of back links your site has and you should aim to get links from authority sites in your area of interest. the more authority sites that link back to your site the higher Google will place you in the search results.
Maybe you should search in DP forum before posting this question here. There are so many explanations about what is backlinks. You can also read trough Wiki pages about this stuff.
hey mat. plz checkout this thread. http://forums.digitalpoint.com/showthread.php?t=1207806 hope u will get your answer from here.
There are many threads in this forum explaining what back links are so check them out. In simple terms, these are links from websites , blogs , forums or social bookmarking sites to your website. They play very important role in deciding your website's PageRank .