Is it necessary to have an SSD? If so what are they particularly used for? Thanks[/quotem
SSD stands for Solid State Drive. It is typically used as a replacement for a mechanical (harddrive/HDD). They are much faster than mechanical drive that relies on a spindle to read information from a platter/disc, using similar technology as a Flash Drive. This helps your computer boot and shut down faster, as well as speeds up loading programs and even installation of programs. Downside is that they are much more expensive in terms of the per GB cost (whereas you can get a 1 TB HDD for under $100, or even under $80 if you catch a sale right now, that same money will only get you up to 120GB for and SSD).
So to answer your question, no you don't necessarily NEED to have one, as you will likely use it as a replacement for the HDD in your build. A common alternative is to have both an SSD and a HDD, installing your OS and programs on the SSD and using the HDD as storage. This allows you to use the SSD for the stuff that will see a real benefit from its speed, allowing you to buy a smaller one to save money, and keeping the other stuff on the HDD. There are a few ways of doing this, but you'll have to read up on that yourself as I haven't delved into it myself.
You should search the forums here on Tom's. There should be a more complete guide about SSDs at the top of the forums under Storage and SSD (the same place you posted this question). Read through it and do a search of the forums for more specific information. There are a lot of people here much more knowledgeable than me.