0 magicka usage

Leaving aside the issue of varying resistances that you'll encounter:

As a rule of thumb, Fire spells will do the most damage, including a chance to cause enemies to catch fire and continue to take damage for a few seconds. Frost spells drain opponents stamina, and slow their movement. Shock spells drain Magicka, and can chain to other, nearby opponents.

In addition, there are several perks specific to each damage type. Above and beyond the pure damage increase perks, the Disintegrate, Deep Freeze, and Intense Flames perks add a chance for an additional effect when casting Shock, Frost, or Fire spells on a target, respectively.

Making it "easier to cast" is relatively straightforward - it reduces the Magicka cost of the spell, meaning you can cast them more often.

This is similar to how things worked in Oblivion. (I'd link you to the UESP page documenting this, but the Site's still getting hit pretty hard from all the Skyrim traffic)

Each spell has a "base cost", which is modified by the following formula:

spell cost = base cost * skill multiplier * perk multiplier * equipment multiplier * dual cast multiplier
skill multiplier = 1 - (skill/400) ^ 0.65

This means that at 100 skill, spells cost 60% of the base cast (further modified by perks and equipment).

This isn't combat related, but as Affe has pointed out below, increasing your magic skills is required to buy high-end spells from vendors.