ID Search mode will show any talkgroup(s) that are being received at any given moment - whatever you hear will be that TG. This is what you'd want for either a brand new system where you have no clue what the talkgroups actually are assigned for, or you can use the ID Search mode sometimes just in case you get lucky and pick up a newly assigned talkgroup you can't find on any published or shared lists, like here in the RR database.
ID Scan only shows and receives the talkgroups you have programmed in, so in effect ID Scan mode is like a "talkgroup squelch" in some respects: it will only let you hear the talkgroups you have already entered into memory.
Basically ID Search listens for anything; ID Scan only "hears" what you've told it to look for.
Have fun, always...