I downloaded Windows 8.1 from dreamspark.com. While I was downloading the file it showed something like a 24 month extension guarantee and also an expiry date in 2015. What does this mean? Will my Windows 8.1 license expire after 2015 or can I use the OS in the same way I use my Windows 7 (perpetual license)? I am pretty new to this stuff, please excuse me if you feel this is silly.
No, the 24 month extension guaranteed only says that you can download the key provided with it for 24 months. You can still use Windows 8.1 afterwards, and the key continues to be valid - you just can't download it from the page anymore.
I myself am using Windows 8 Pro provided by my college through DreamSpark. the 24 months time specified is the time starting from the date of creation of your account within which you are permitted to access the product keys. Colleges generally provide 2 year subscription after which you have to get it renewed from the college's dreamspark administrator.