I can see that GD library is for images. But I can't see differences between PECL and PEAR. Both have authentication. What are the main differences between two? Why don't they combine them?
PECL stands for PHP Extension Community Library, it has extensions written in C, that can be loaded into PHP to provide additional functionality. You need to have administrator rights, a C compiler and associated toolchain to install those extensions.
PEAR is PHP Extension and Application Repository, it has libraries and code written IN php. Those you can simply download, install and include in your code.
So, yes they are similar, but yet so different :)