There are several humans which can transfer into titans at will. What about the normal titans? Hange said that titans do not weight as much as they should. Their only vulnerable area is where the real human body would be for shape-shifting Titans. They also look like humans, are there humans inside normal titans as well?
This was explained titans were humans at one point. The ones from the Ymir's village were turned into them against their will. http://shingekinokyojin.wikia.com/wiki/Titans
It was also hinted at by experiments with Eren that the more he uses shifting without resting the more he merges with the titan.
Ymir also wandered aimlessy as a titan without ability to shift or control her actions for a while, indicating most titans are in the same situation as her.
Although the manga hasn't actually confirmed this, but here's a theory.
Honestly, don't read the manga spoiler if you don't want it. You will regret it.
As we already know, Eren and Annie are humans who can turn into Titans. In the manga, Ymir, Reiner and Bertolt are revealed to be able to do the same. There is also this Beast Titan, who is very intelligent and can speak. It is also able to turn humans to titans through some unknown means and give them orders. The manga also reveals that there are titans inside the walls. Maybe that would explain the last scene after the ending in the anime (the Colossal Titan inside the wall Sina).
So my theory is that the Beast Titan can convert humans into titans. Most are stuck in this form, and their human bodies eventually merge with the titan's and they become full titan, thus become unable to return to human forever. Some are able to break free, and they become Titan Shifters
The most reasonable explanation is that the Beast Titan turns them into full titans. If you read the manga, it says the Beast Titan can turn humans into titans without a serum. They revealed that a SERUM turns humans into titans. They cannot break out of this form, unless eating one of the nine titan shifters. Only 9 can change form, no more than that.
Regular titans have had their weak area cut into. The visible result was that there was no human in that or any other part of the body. The weak area does, however, correspond with the size of the person's spinal cord but that isn't their either. The ability to transform back into human is a special power special titans have... they can obtain it by eating other transforming human/titans.
No, the regular titans, not the titan shifters, were humans who were injected in the back of their nape with some sort of liquid, most presumably titan spine fluid, by the nation of Marley. It's kind of like a punishment, for example Ymir was taken in by a religious group and got turned into a mindless titan as a punishment due to the religion going against certain things that are classed illegal. However, the beast titan can turn people into mindless titans so he also plays a role in this.
Titans don't weight that much because technically they're a fabrication of the normal human anatomy, meaning they only use what they have, the human body, to make a titan out of it, so as the human body doesn't weight as much as a titan would look to weight, they don't look their weight.
My guess would be that when you would be turned into a titan, your body will change and become something new, a titan, and this is reversible due to the fact that if a titan eats a human who possesses one of the (now 8) titan abilities, it will turn back into a human but it will possess that titan ability. The only reason titans eat humans is because all they know is that eating humans is the only way to pretty much end what they're going through.