DO NOT READ IF YOU WATCH ANIME AND DO NOT WANT SPOILERS.
Attack on Titan is a very famous anime based on a manga , first season ends around the 33 chapter of the manga and the second chapter starts with 33-34 chapter of the manga.
There are some shortenings in the anime to that make manga more explanatory , for example you can understand from manga that titans turn into a sleep and when they do so they cover with some kind of strong material , even from the first episode they try to figure out how titans formed the wall and what was the reason for them to do so. You may be totally surprised by this plot twist and THAT IS UNDERSTANDABLE.
I didn't understand why the training kids didn't take any weapons with them , there where not 1000 titans just 8 and they would not survive without weapons because as they say it is no time to put them on, like without them they have great chance, the titans where not at the windows or in the building they where far away on the path , how long to grab them with yo on the horse ? At least put your blades in a pack or something , that moment just made no sense , I think is kinda plot rush.
The walls are made of hardened titan bodies and Eren as you might have guessed is super special , he has the power to command other titans , summon them to attack , not all of them but the less developed ones , at least at the beginning .
By the general idea there are 2 types of titans at least at the point I got , one of them can transform when it wants and other are transformed people . If a human transformed into a titan will eat a human that can transform into titan , he or she will be able to transform into human.
Eren inherited the power of one of the 9 titans that where spoon by the first titan , the name of his titan is Attack Titan.