What are differences between PECL and PEAR?

PECL is a repository of C extensions for PHP. Those extensions are usually installed via the pecl command, which is an alias for pear, with the default channel/repository set to pecl.php.net.

PEAR is multiple things:

1. The package installer (pear command) that is able to install packages from different channels (repositories), including pecl.php.net. Many consider composer the successor of the pear installer.
2. The package repository http://pear.php.net/packages.php with ~600 libraries. Many of them are composer-installable.
3. A PHP coding standard that is available with PHP_CodeSniffer.

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 :)

pear is a php classes written in php Whereas pecl is extensions which not written in php and need compiling .

PEAR as-such is not a coding standard. Instead PEAR "follows" a set coding standard that is encouraged within the PHP Community. This is however not the only coding standard that PHP users can follow, the ZF, for eg. recommends a slightly different standard.