Naturias and X-Sabers are what are called "archetypes" of monsters. "X-Saber" monsters all have "X-Saber" or "XX-Saber" in their names. "Naturia" monsters all have "Naturia" in their names. Other archetypes include Lightsworns, Gladiator Beasts, and Blackwings, just to name a few. A good majority of Archetypes are designed to work together and support each other while in a deck. And of course, each archetype has various Spells and Traps that support them.
Use this link: http://yugioh.wikia.com/wiki/Main_Page
The Yugioh wikia is extremely helpful. It contains almost every card released in both the TCG and OCG, rulings, tips, deck types, and so on. Just look around.
If you want to get back in the game, I highly recommend buying 3 copies of the Machina Mayhem Structure deck.
They're not gonna be hit too hard by the Sept. 2010 Ban List, so Machina Gadgets is still a decent deck type. With 3 copies of that + a few staple cards (Mystical Space Typhoon, Mirror Force, ect), you'll have a fairly competative deck in no time.
It'll definetly be your best option for getting back into the game.