It is generally advisable the use of jargon's and abbreviations be avoided in your statement of purpose. So if I would like to mention a few MOOC's(which stands for Massive Online Open Class offered at portals like Udacity,Coursera,etc) and the flow of my SOP necessitates the use of the word "MOOC", what do I do? Is the term "MOOC" known widely (which I believe is true) or do I have to mention "Massive Online Open Class" (atleast for the first time). This usage could be decisive in situations where there is a strict limitation on the word count.
One of the most important rules of writing is know your audience.
When you're writing a statement of purpose, you are addressing an audience of experts in your general field. So, everyone in your field will know the basic terminology and jargon that is common to people throughout your field.
Therefore, if you are using a specific term that is not in standard usage throughout your community, you should not assume that everyone will know what it means. Therefore, for the first occurrence, you should write out the term in full. After that you can certainly list an abbreviation.
On the other hand, if the term is normally or better known as its abbreviated name, then you don't necessarily need to expand it. In my field, for instance, there is something called the Bogoliubov-Born-Green-Kirkwood-Yvon hierarchy of equations. However, because it's such a mouthful, it's pretty much universally called the "BBGKY" hierarchy. In such a case, providing the full name isn't going to make much of a difference; if your audience doesn't know the abbreviation, giving the full name won't hurt. But this is such a narrow case of exceptions that in general it's a good idea to provide the full name at least once.