Primer on SSD and internal hard drives

I recently acquired a streamer /DAC that supports an SSD or internal hard drive.  This is new turf for me and wonder if I could get some basic guidance.  Please pardon my complete ignorance on this topic.

So what is the difference between an internal hard drive and an SSD?  Is one preferable?

The manual suggests using either a 3,5" or 2,5" drive. Does size matter ? ( no pun intended)

Are there sonic differences in the different available brands?  

Again, pardon my lack of knowledge on this topic and thanks for your help.

Mike