Ok
This was my first bus terminal in Turkey and it's huge. It's not a bus terminal in the traditional sense of going into a building and having all the various bus companies in small cubicles with destinations, times etc listed. It's a giant oval shape and each company has its own shop front with the biggest companies having multiple ones. So shopping around is a little trickier.
It serves everywhere so wherever you are going you will find a bus.
It's very confusing and messy but practical.
This is a huge terminal for buses. Find your bus company first and your destination, if not you'll might get confused. It's not clean and quiet messy. But it's ok since you're in Istanbul.
You can go from it to any states in turkey by bus
As you have probably read before, this place is a mess, you'll be in a bad situation if you do not have some one to pick you up because you'll have to use coins to pay the ticket for the metro! And around there are a lot of dealers, pickpockets, weird people, homeless people, ilegal exchange stores so be careful and get out as soon as you can
So unorganised. Crowded. Unsafe. Try to be away and prefer another transporting system.