Over 80% of all comments submitted every day to .wordpress.com blogs is spam. Please scroll down and read "5 things every blogger should know about spam". The very worst of all spam contains malware and viruses that could bring gown many site if opened so that is not sent to us.
You can go here Dashboard > Comments and then click the Spam link
Then you can carefully examine what you find and approve any comments you are sure are legitimate. Do be cautios because posting spam comments has a negative affect on your blog's authority and pagerank. For some tips on learning how to spot spam this article may help.