According to Joomla official website, Joomla 2.5 will reach its end of life on Dec 31, 2014, that means all Joomla 2.5 websites will not receive any updates including security updates. If your website still runs Joomla 2.5, make sure that you upgrade it as soon as possible. If you are planning to start a new Joomla site, make sure that you choose Joomla 3 to avoid unnecessary upgrade later.
You should be careful when choosing a 3rd extensions to install into your Joomla site, avoid all Joomla 2.5 compatible only extensions: there is no point to choose them, we’ve seen a lot of users who stuck with Joomla 2.5 because of some 3rd extension developers do not upgrade their extensions for Joomla 3.
Below is a nice tutorial which cover the process of upgrading from Joomla 2.5 to Joomla 3: