The Starter Pack includes:

Five Heroes – Zagara, Sonya, Li Li, Jaina, Zeratul
The Ronin Zeratul skin
The Golden Tiger – a unique mount only available with the Starter Pack
Already got some of this stuff? No problem. Each of the seven items is included as a separate code, so you can simply gift that spare Zeratul to a friend or team-mate.

Heroes of the Storm is available to play for free by anyone with a Battle.net account. As you play, you keep earning gold that you can use to buy any hero in the game. The Starter Pack is just a great way to start your collection of Heroes, skins, and mounts. Other discounted bundles are available through the game client, so be on the lookout for other bargains.