Pokemon Emerald may look like Pokemon Ruby/Sapphire, but there are tons of changes inside it.
Same as any other Pokemon games, you start as a kid in a small town with nothing special, then you saw a professor attacked by one of the weakest pokemon in the game, you helped the professor with one of 3 pokemon stored in the pokeball lying on the ground.
After that, you start your journey to be the greatest pokemon trainer.
The core gameplay of Emerald is same as other Pokemon games, you catch pokemon in grassy areas/sea/cave, level them up, fight some trainers on your way, kick the bad dude's ass, fight Gym Leaders, then the Elite Four.
Pokemon are classified into types like water, fire, grass, electric, rock, etc.
Every type has a weakness and strength, so theres no "ultimate" type. Some pokemon have 2 types, which could be good or bad, depending on the type combination.
There are more than 350 pokemon you could get, but 1/2 of them requires trading with another pokemon game.
Emerald has many activity to do when you're bored of random fighting outside. There are contest, which test Pokemon's beauty/coolness/smartness/toughness/cuteness. To increase these things, you must give them Pokeblocks. You can get Pokeblocks by blending berries with your friend or NPCs.
Another fun thing to do is the safari zone, it's like a pokemon zoo, you pay money, go in, catch rare pokemon using special type of pokeball.
After beating the Elite Four, the credits roll, and a big "THE END" appears. Well, they're lying, because that's just 50% of the game. There are tons of thing to do in Emerald after beating the Elite Four.
First, there is this Battle Frontier which are very hard challenges, where you must fight strong trainers and not losing. This is one of the best part in Emerald. Every time you win a battle you get some Battle Points, which could be use to buy super cool items.
Second, you can fight the Elite Four again for experience points. And a new addition in Emerald, you can fight Gym Leaders again, up to the 4th time. When Gym Leaders are ready, they will call you.
Third, "gotta catch 'em all", yeah, fill the Pokedex.
Overall, Emerald is a good game, despite sharing many same features with it's predecessor, Ruby and Sapphire.
If your interested in Pokemon and haven't played Ruby/Sapphire before, give Emerald a try. Even if you've played Ruby/Sapphire, Emerald's still a good game with tons of new features