The point of copyleft is to guarantee a work is free to study, copy, modify, and distribute for all. However, sharing a work with the conditions of a copyleft license is different from just completely giving the work away. If you give a work away, the other party may create a derivative work and then claim copyright on that work. For example, they might make some modifications to your work and claim copyright on the result; they would then be able to keep other people, including you, from copying their modified version.
The point of copyleft is to share the work with others, while at the same time preventing anyone else from privatizing a modified version of your work. Any derivative work must be just as free as the original.
The simplest copyleft license would be this:
As a bonus, you could add:
The GPL is specialized for software; in order to "modify" a program, you need access to the source code.
The GFDL is specialized for manuals; it makes sure that the sections concerning the thing documented remain modifiable, while allowing authors to impose restrictions on other parts of the book (just like traditional copyright allows them to).
A copyleft license doesn't allow you to do things to the text that are illegal. You cannot take a statement by X, distort it, and still claim that it was X who wrote it. You also cannot take a statement by X and claim that it is yours.
Most copyleft licenses require modified versions to be distributed under exactly that license. This prevents the combination of source materials under two or more copyleft licenses into a third work.