If you set it to record tracks to the card, it will record a gpx file for each day, named by the date. If you are riding at midnight, the part of the track after midnight will be in a separate gpx file, and it will be up to you to join it up with the pre-midnight part of the ride afterwards. The amount of data logged is limited only by the amount of free space on the card. Saving points at 1 per second, a day's ride uses something like 3-4Mb.
Data recorded is time/location/height.
Logging data to the card is in addition to the normal logging to internal memory (below).
If you don't set it to record to the card, it will record in internal memory as the "active log", with full time/location/height info. The active log is limited in size, and if you do a long enough ride recording points often enough, it will wrap (depending on setting) and you will lose the start of the ride. You can save the active log to a named track at any time, but doing this strips out the time and height info, and may also discard some points.
Note that the active log and the named saved tracks are the only ones you can navigate along. You can't display on the GPS or navigate tracks that are saved on the card.
I'd suggest always saving tracks to card.