+1 on Zardoz's comments. Folks can be very polar when it comes to optics so take opinions for what they are. I would also suggest checking out different manufacturer's websites as they tend to have some good general optics info as well.
Short answer to your Mil Dot question - it is a ranging reticle using miliradians (the "mil" in mil dot) as a means of estimating range to target based on the size of what you intend to hit. Premier Reticles' website (www.premierreticles.com) has an excellent explanation of this reticle type, as well as decent prices (and great service) on Leupold scopes.
As far as what type of scope for different applications, you'll get a lot of opinions on that one. My eye sight is not that great, and I think that higher magnification helps my confidence on 200+ yd shots, so I tend to lean towards a max magnification of 14x. I also like variable magnification because I typically use my rifles for target shooting and hunting so being able to dial down to 4 or 5x magnification is of some value to me. For others, the simplicity of fixed magnifcation is preferable, or, they may shoot at very long distances and have no need for lower magnification. I guess the point I'm making here is that are no hard and fast rules - some general guidelines maybe, but it is all about personal preference. Some things I would consider before I make an optics purchase are - your budget, distances you will typically shoot, max distance you ever plan on shooting, intended targets (paper, tin cans, animals), and size/weight and how that impacts the utility of your rifle.
Take your time and do a lot of research. It is a purchase that you will have to live with every time you shoot your rifle. Also, plan on spending a few bucks on decent rings.
Hope that helps.