Record Sleeves - Which ones and where?

Sorry for being dim, but what are outer sleeves, why are they important and how do they protect the LP from wear, tear and dust? I've heard of plastic covers used to protect the album cover, but never heard of an outer "sleeve" that could protect the vinyl record itself. Thanks.

Thanks guys. Very interesting. Now I have about 800 outer sleeves to buy. Sheesh. When does it ever end. I've been buying a ton of inner sleeves since I got a used Nitty Gritty Record Doctor a few months ago, which I absolutely love, by the way (brought a whole new clean experience to my vinyl). I had never really thought about outer sleeves. You're right, with just inner sleeves, its like only doing half the job.