When it comes to playing Magic: the Gathering, resource management is always the name of the game. Whether you're defending your dwindling life total, counting up your lands to cast a game-changing spell, or wishing that you had a few more cards in hand, you're dealing in game resources. When you're still learning to play, the only resource you tend to concern yourself with is Mana. Mana allows you to cast spells and spells allow you to win the game. That seems simple enough. But what happens when you run out of spells? What's the best way to handle a swarm of attacking creatures?
These other resources tend to be relegated to more advanced play, though I think that's just to keep things as simple as possible for newcomers. It doesn't take much more wisdom to play with your Life Total as a resource than just Mana. It's just one more dynamic to watch during the game!
Using your life total as a resource can be intimidating - scary even - as your life total shrinks and you're left wondering how you can be so close to defeat. It's simple enough when you think about it, though. An opponent is at 2 life while you sit at 3. They have no cards in hand and are attacking you with a 2/2 Grizzly Bears with no other creatures in play. Do you block and trade with your own Grizzly Bears or do you take the damage? You can survive a hit from the bear. The opponent, however, cannot. It's folly for them to be attacking here, but it's a demonstrative scenario, so we'll let it fly. If you let the bear through, in order to get your own bear through, you're using your life total as a resource. You have opted to take 2 damage in order to deal damage to the opponent and thus win the game. Of course, if they had a Giant Growth or a Lightning Bolt in hand, the situation would be a little different, but we'll get to that later.
Some cards look to use your life total more directly. Necropotence, for example, effectively lets you pay life to draw cards. While Channel lets you pay life to add mana to your mana pool. These direct 1 to 1 exchanges are so powerful that both of these cards are banned in the Legacy format. Using Channel, for example, means that you can use your life total to burn your opponent with a Fireball, potentially as early as Turn 1. This does not make for a very interesting game.
Phyrexian Mana is a mechanic in Standard that seeks to use life directly as a resource. Vault Skirge, for example, costs 1 colorless and 1 Phyrexian Black mana which can be paid with either 1 Black mana or 2 life. That means that you can drop it first turn! A 1/1 with Flying and Lifelink is not bad for a virtual 1-drop!
This is a trickier subject, so I'll direct you first to the Wikipedia article which details it, in case you'd like a more in-depth analysis. If you've read the article, then this should mostly be familiar to you but, in the interest of inclusion, I'll recap much of what it said. In short, each card you have access to, be it on the Battlefield or in your hand, counts toward card advantage. Each player begins the game on equal footing with 7 cards. That is, of course, unless you've Mulliganed, in which case you're trading card advantage for a new opening hand! If you choose to play first, you're sacrificing card advantage again for the opportunity to take the first turn.
If you cast a spell that allows you to draw 1 card, like Think Twice, you're just breaking even (until you cast it again with Flashback, at least!) If you cast a spell that allows you to draw 2 cards, then you're gaining +1 card advantage. Similarly, if you Doom Blade an opponent's Craw Wurm, you're breaking even or 'trading'. If you cast Day of Judgment when they have two creatures in play, you're getting a 2-for-1. You've destroyed two creatures with a single card from your hand. Finally, and slightly less prevalent, there are discard spells. Spells such as Mind Rot force the opponent to discard two cards, giving +1 card advantage to the caster. Other discard spells, such as Despise, allow you to look at the opponent's hand and choose what they will discard, which is arguably more powerful even though it merely trades 1-for-1 with an opponent's card.