Vainglory was first featured at the Apple event announcing the launch of the iPhone 6, where Apple showed off this game because it took advantage of Metal, Apple's new graphics platform.
After playing a prerelease version on both the iPhone and iPad I can tell you it's a great game for longtime battle arena fans as well as those wanting to learn the ropes of the genre. It also has some of the best graphics I've seen in a mobile game.
As I mentioned in the beginning, Vainglory is an example of a MOBA. For other examples from this genre, you might recognize 5-vs.-5 MOBAs including Heroes of Order and Chaos on iOS and League of Legends, Smite and DoTA 2 on PC. It's important to note right off that there is usually quite a learning curve to this genre of games, but with Vainglory, a simplified map that only has one major path and a maximum of 3 vs. 3 players makes it a lot easier to pick up and play. It also has two helpful tutorial levels that layout the gameplay and important map points so you can get up to speed quickly without feeling too intimidated.
In other words, Vainglory is a great way to break in to the battle arena genre in general and will help you branch out to other games in the genre if you like the gameplay. But experienced players will like it too because, even with the simplified map layout, you'll still be able to develop unique strategies to help your team win.
Picking and advancing your hero
Once you're through the training levels you'll be ready to try your hand at a real 3-vs.-3 match. The game is very good about matching players, so you should get into a game quickly with people of your skill level.
Next you'll need to select a character that you'll play during that game. There are 10 heroes to choose from with a mix of ranged and melee attack capabilities. Once you make your selection, you'll enter the game soon after.
As you progress through a game, your character will level, letting you unlock and upgrade your core abilities. You also can buy items back at base that complement your abilities to make your attacks and other skills stronger. When at the store, it's best to follow the Recommended tab to buy items until you learn the ropes and want to experiment with other character builds.
One thing to point out here for beginners: You are only advancing your character through one game session. Once the game is over, all your upgrades are reset. MOBAs don't have persistent characters that grow over the course of several games (like RPGs) -- they only advance within that particular game session, so the challenge is figuring out how to perfect a hero with the right skills and items in the shortest amount of time possible.
In a standard 5-vs.-5 MOBA like League of Legends or DoTA 2, there are three paths on the map called "lanes" and your team is challenged to blow up the defensive turrets in those lanes before destroying the enemy base to win the game.