Have you heard people complain that Windows computers get more viruses than Apple computers? There are many reasons for that. One of the most common is that there are just so many more Windows computers out there. That's why they make a much more inviting target.
The same is true with WordPress sites. More than 25% of all websites run on WordPress. Hence, they are a very attractive target for malicious users. It's common for WordPress beginners to end up with hundreds of spam comments on their posts.
The solution? Akismet.com. They provide a great spam-blocking service. You'll need it because around 83% of all comments are spam.