Plain SVG is a file format of inkscape and other programs, made for compatibility between svg drawing programs, to strip of all programrelated information that is no use in other programs and because of filesize. It is a human readable format, because the filecontent is saved as ascii-text.
Another option is the svgz file format which is a normal svg file packed with gnu-z. That shrinks the filesize, but produces a binary which in not human readable as it is.
It is a good idea to save your work files in svgz format, to save space on your harddrive. Inkscape and some other programs as Ksvg can read svgz files without a problem or timedelay.