Microsoft's support for Windows Vista is rapidly coming to an end. The operating system will no longer be supported come April 11, which means those who are still on the platform will stop getting security updates, non-security updates, and assisted support options.
While it's still possible to use Vista after support ends, it's not the best possible choice. You'll raise the risk of running into security problems and the majority of people will generally be happier with a more recent Windows version anyways.
NetMarketShare says Vista has a market share of 0.78%.