Composer is nothing but a tool which is useful for dependencies management. Composer supports only 5.3.2+ version of PHP. Now a days most of the frameworks are using PHP composer. This idea comes from node's npm and bundler from ruby. You should be familiar with composer before using zend,cake and magento etc. You can download the composer.phar in location /usr/local/bin/composer to set it globally or you can download it your project root folder.

curl -s https://getcomposer.org/installer | php

After that you need to create composer.json file to set the dependencies for your project. Many frameworks are providing inbuild modules for general purpose. You can use the composer to download the modules files in vendor folder. You can simply update the composer using command

php composer.phar self-update

You can simply install the dependencies which is set in composer.json in root of your project. This is the command

php composer.phar install

Other question is raised that what is the need of it. We all knows day by day PHP is improved by their owners. New versions are launched by different framework owners. Its all about the coding style.

Composer also provides autoload capabalities. You can autoload the specific files in your project to fullfil requirement.