granca- excellent question!
Rebanada, rodaja, tajada, and lonja all mean slice in English but in Spanish the distinction is with what you are cutting or slicing (primarily) and then some people use the different words synonymously regardless what they are cutting. Some speakers are more specific than others and stick to the distinctions. However, in general, the words are used freely and some are more common than others, also regional usage is a factor.
For example, rebanada is usually for bread, fruits or even pizza. Rodaja is usually for potatoes, tajada for meat and can be very general in usage. Lonja is the most specific in my opinion in the group of four words for slice that I mentioned above and is used to refer to thicker slices of meat and even cheese. The thickness and thinness of the slice also determines which word to use and there are more words. I only mentioned four common ones.
However, as I mentioned above if you interchange them there's no issue with comprehension. In my opinion, rebanada is the most common and I use it the most when referring to a "slice" of something.
My two "rebanadas" on the topic.
-M@rianoP.