Pokemon Emerald is the third game in the Hoenn generation, like how Yellow was to Red/Blue, and Crystal to Gold/Silver. It includes many additions to the previous games, such as the Battle Frontier, a new set of difficult challenges, a new gym leader, and a new Champion. You can choose a second starter, one of the Johto trio, after
completing the Hoenn Dex, and many new Johto Pokemon are available to capture in the Safari Zone once you have defeated the Elite Four.
Both Kyogre and Groudon can be captured, as well as lot more legendary Pokemon including Mew, Lugia and Ho-oh (as long as you have the right tickets). This game also features the speed variation of Deoxys.