There seems to be a new style of naming for this particular reaction, nowadays, teachers are calling it -- a metathesis reaction. You can seach this forum for more on that name. It's come up often lately. Why this name is preferable to double displacement, that I don't know. Maybe your book or lab notes has a hint. This is a typical outcome -- when you have a definition that you don't understand, only your book, class notes, or instructor can give you the best answer. Because it's often unique to your class.
Now, on to your experimental write-up. It's hard, when you're just starting out, to find out what your sources of error are. It really does depend on what, exactly, you did. For example, what did you measure? Measurements are what's usually in error. I mean, it's also an error if you grabbed sodium chloride instead of sodium sulfate, but that's not a measurable error, the reaction simply won't go -- all or nothing, as it were.
For the fun of it, can you write you're experiment's reaction using a balanced chemical equation?