I am British, but old, and "save the date" is a fairly new phenomenon, so my info is gleaned from years of reading MN!
STD means, this is the date we plan to marry, and we would like you to be there. It is not an invitation, and requires no more acknowledgment than "How exciting!" and sticking a note in your diary.
You might then receive a formal invitation to the whole of the wedding ceremony, meal, and evening do.
Or...
You might then receive a formal invitation to just the evening do.
Or...
You might hear nothing further at all, in which case you are faced with the excruciating dilemma of asking if you're invited and it got lost in the post, or sending a card with a passive-aggressive tenner included.
Having received an STD, your life is now on hold - do not plan holidays, get pregnant, fall ill, or lose your job. And start saving every spare penny in case the actual invitation is to a three day event in the Carribean, no kids.