Most of the info has already been shared... As has been pointed out, there are 8 elements (not 6), and having all 8 means you can access all parts of the game. Funny how EVERYBODY, including the game, has a "If you have a Giant" attitude, when one comes with the game, Tree Rex! Since you say you got the Starter Pack, would that include the figures, such as Tree Rex? If so, you have the Giant part covered. Between the 8 elements and having a Giant, that's the bare minimum to be able to access all locations.
As has also been said, the Legendary characters are exclusives from Toys R Us. They have similar stats, but a different colour scheme (which shows in the game as well, not all variants look different on screen), which always seems to be black or dark blue with gold accents. There were also Legendary characters which were released with the first game, Skylanders: Spyro's Adventure (SSA here on the forums).
There are also other exclusives from other stores and in certain packages. Life will be easier for you if you satisfy yourself with the normal versions, the variants can be hard to find and hard to get, and expensive outside of a store.
Note that in the game you can get preview videos. Pause the game, select Collections, and select the first button, with Tree Rex's face on it. It will show you a grid of the 8 elements. Pick an element, you get profile pics of all the characters (from both games) in that element. Anybody with a little Play icon, you can see their preview video. Characters who came out with SSA, you can see them right from the start. New characters (which will be the first two profile pics, the Giant and the new non-Giant) you have to find their Soul Gem in the game first.
If you go to the store, a lot have a portal set up in a display. If you put a packaged character on their portal, the TV will show these same preview videos.
Single packs tend to be $10, triple packs/adventure packs/battle packs tend to be $20 (so for triple packs you're saving $10). Adventure Packs are bonus levels you can play in either game, and comes with two magic items that can affect things in the game (such as a potion which heals). The level items act as a mega-attack magic item while playing. Battle Packs are two characters and one large magic item. I only know of two, the Dragonfire Cannon and the Scorpion Striker, I only have the former. The cannon follows you around like a pet, firing on enemies when they're around.
If you want to know what's available (though what's available in stores is a different story), at the top of this page is a list of icons for different Spyro games. Click the one for Giants, on the left will be a list of levels (clicking a level - Chapter - name will give a walkthrough for that level). Under the level list is more related links, including Packs. This shows what character packs have been released, including variants and where they might be exclusive to. Doing the same for Spyro's Adventure will show you what was released for SSA. Together, that's everything that's out.