M1 Line (Aksaray-Havalimani)
The M1 line is especially useful, as it's the line that connects the airport with the centre of Istanbul.
M2 Line (Taksim-4.Levent)
The M2 line really is of very little touristic interest.
Just like the trams, the Istanbul metro runs from 6 am to midnight. The frequency, depending on the time and the day, varies between 5 and 10 minutes.