An exact match domain is a domain that exactly matches the keyword you are trying to rank for. For example, if I am making a website that is trying to sell Red Rider BB guns, the exact match domain would be www.RedRiderBBGuns.com
Having an exact match domain doesn't really have anything to do with spammy sites, any site can be spammy, but whether they have an EMD is irrelavent.
Having the exact match domain for a product or service is valuable for SEO (Search Engine Optimization) because it lets the major search engines know that your website has information that is relevant to the keywords in your domain name.
While you would still need to do other SEO to rank your site well, if you are competing against another website for the #1 spot in Google for your keyword, and you have done the exact same amount of SEO, but you have the exact match domain and they have something like www.BBgunsRUs.com, 9 out of 10 times you will get the #1 spot. The only reason for Google to give them the #1 spot over you is because Google likes to do irrational things sometimes.