Ooh! Ooh! I know this! Excuse me for I am about to unload my limited nerdy sleeve knowledge on you.
There are a few types you can get that I know of. (Note: L size photos are 3.5x5 so always look for that when buying plastic sleeves. I use Amazon, but I linked to the companies websites below.)
One is put out by <a href="
http://www.c-lineproducts.com/">C-Line</a> and the other is <a href="
http://www.ultrapro.com/division.php?d=p">Ultra Pro</a> and <a href="
http://www.bcwsupplies.com/products/pro ... s/">BCW</a>. Ultra Pro and BCW are basically the same, but Ultra Pro also offers horizontal L pages and BCW offers a ton of weird sizes. (It's mainly for collectors.)
(H!P shop sells sleeves with collection photos, but I have no idea what brand it is.)
There are many reason people choose their brand. I use C-Line and BCW. I chose BCW in the beginning for myself because I got a good deal on the price and someone gave me a large package of used ones. I also have C-Line sheets for my 2L photos and trading cards.
I find that the C-Line plastic is very thin and flimsy, where as the BCW is stronger and doesn't bend as easily. BCW is also archival so it protects the photos better, not sure about C-Line. The drawback to BCW is that there is a ton of room surrounding the photo, so it's not a true 3.5x5 fit for the photo, were as the C-Line sheets hold the photos better and there isn't a lot of movement between photos when they are back to back in a sheet. I liked how C-Line sheets fit my 2L (2L size is 5x7) photos better then in the Ultra Pro sheets, except that the photos go in on the side rather then the top, like every other sheet I own! But overall I stick with the BCW for the L photos because I think the plastic sheets sit better in the binder and I like the sturdy plastic. I can live with the photos shifting around, I just make sure when I put them to be careful.
Other photo sizes include Trading Card (Derp, these are easy to find sleeves for!), Postcard (4x6 - Standard American Photo Size,), A5* (8.3x5.8 - Crappy F'n Size! So much hate!) and Pinup A4* (11.7x8.3 - Newer pinups come with sleeves, older ones don't.)
*The A5 size is relatively new in the H!P world and I hate it because there is no A5 sleeves available. (They sell a binder on UF-Online but I'm not paying that! I end up just putting the photos in the 8x10 sleeves and dealing with it.)
H!P gives you sleeves with the A4 pinups, but you need to find either a 2 ring or 4 ring binder and those are expensive unless you know someone in another country (Europe/Asia) willing to buy and send you one.
Okay I'm done now. I hope this was a helpful and informative post.
In the end, since you're new at this, go with the cheaper option and if you find yourself turning into a collector, you can use your judgement as to which you like better.