DISCOVER FINANCIAL SVCS and other strange stuff

Just got my credit report from Experian and I noticed that "DISCOVER FINANCIAL SVCS" requested my credit history about six times over a three month period last year. I've never had a Discover Card or even ever applied for one, so I'm curious as to why this is appearing. Also wondering about "FIRST USA,NA" and "HSBC NV" which have also both made multiple credit history requests without me seemingly asking them to do so. Does this have something to do with junk mail credit card "approvals" that they send out? Is it something that I should be worried about affecting my actual credit score?

Thanks!