Silver Maple

http://en.wikipedia.org/wiki/Silver_maple

 Acer saccharinum, not to be confused with A. saccharum, which is a sugar maple (syrup, anyone?), I think this tree's most notable characteristic is it's silver-underside leaf.

http://en.wikipedia.org/wiki/Silver_maple

The lobes are a little more defined, I think, than the generally-three lobed, balanced shape of the red maple (Acer rubrum; I'm trying to memorize this sort of thing.)

http://en.wikipedia.org/wiki/Silver_maple

The angles in the leaf are more defined, but not necessarily pointy (I've seen pointier leaves in other maples so far).
The sinus (veins) are deep, easily noticeable.

http://en.wikipedia.org/wiki/Silver_maple

It's a little hard to tell, but if you really look at it you can see the alternation of the leaves. They don't bunch up, but hang out in pairs.

The tree grows quickly, and generally has brittle wood. That means it tends to lose limbs. People like the way it looks - and appreciates the speed of its growth - which is why it's used often enough. The roots like to seek out water sources, though, making it similar to the willow with how it wants to get into water lines and such. 

The limbs 'split' early in growth, meaning that there's a little base at the bottom where a couple-few large limbs will branch out at an angle. This puts stress on the tree. Strong angles and weak wood generally mean that when the tree's limbs get heavy, or if a storm comes by and stresses it enough, it'll drop those limbs - and the ones I've seen weren't small. Pruning early can theoretically prevent this, but most people wouldn't do that. 

*Note: I'm not exactly adept in this yet, so this MIGHT not be a silver maple. The internet says it is. I'd like to say I'm using the trunk as an example of what I'm about to talk about, not necessarily to prove what the bark looks like.

http://cdn.c.photoshelter.com/img-get/I0000K9jr167PcHQ/s/750/750/tree-trunk-826.jpg

This trunk has some obvious splitting points. It's not quite as much of an extreme angle as it could be, so it may live longer before dropping a limb, but it's bound to happen.

The one I saw on campus had a split of three trunks from the base, and there's almost a line in the bark where one can tell where the stress will split the wood. It's still a nice tree, and provides good shade, so I personally think it was a nice choice. I just hope people understood that the tree won't look exactly like that for hundreds of years.

http://en.wikipedia.org/wiki/Silver_maple

The samaras are a little more squat and bolder in color, from the looks of things, than reds - but, so far I've learned to be careful with trying to depend on consistencies so desperately. They do seem more rounded, though, and in general it appears to be pretty easy to tell the samaras apart from the red maple's.

http://en.wikipedia.org/wiki/Silver_maple

 The bark is grooved but not as extremely much as I saw with the red. It seems to be a little more delicate, and it is, in a sense. The wood grows pretty quickly. The tree loves water, and I can understand why: it grows well in the midwest (one of few, it seems, that doesn't mind growing there) and I'm sure water is a different matter altogether than it is here, as far as acquisition is concerned.

http://www.dcnr.state.pa.us/forestry/commontr/images/SilverMaple.gif 

I actually rather liked this diagram for the general perspective of trying to understand how this one differs from the others. People can tell trees apart by just the growth of a stem, and that fascinates me. Looks like a stick to me, at the moment, with some little buds and such.