Basically the grand exchange is a massive market where you can sell any item and buy any item, although there are some restrictions to what you can buy, like 4000 of each type of rune per 4 hours or something like that, and you may notice that the prices of items, rune for example will change very often due to people or merchanting clans buying lots of that item making its price go up. There is also a new skill called dungeoneering, which is where you can enter a dungeon on your own or with a group of people, in the dungeon you have to explore it and may encounter monsters, puzzles, traps and even a boss at the end of the dungeon. as an extra note you cant take any of your own items into the dungeon and you make, get, collect your armour weapons while in the dungeon. I hope some of that helped ^^