Any train going away from it's base zone/station is called as UP; and any train coming back to it's base/home zone is called as down.
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UP trains usually have odd last digit in train number; while the DOWN trains have even.
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E.g.,...
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12009/12010 is homed at BCT (however station in same zone) hence, going away to ADI it is called 12009, while returning it is 12010.

Down refers to a train travelling away from its headquarters (i.e., the homing railway) or from its Divisional headquarters, whichever is closer. Up refers to a train travelling towards its headquarters or divisional HQ, whichever is closer. Eg, 2903 DN Frontier mail is down travelling away from its HQ (Bombay) and from the division that homes it (Bombay division) and similarly 2904 UP refers to it travelling back towards Bombay, its HQ and division. Usually the numbers for the "up" and "down" trains differ by just 1.
As for the Shatabdis, the odd number indicates the Down run and the even number for the Up run. E.g. New Delhi - Bhopal Shatabdi (the first Shatabdi) is numbered 2001 DN (away form Delhi)...
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and 2002 UP (towards Delhi).
There are numerous exceptions to this scheme! Beware if you are trying to find method in the madness. Numerous inconsistencies and oddities arise near zonal or divisional boundaries. At Howrah, the Kalka Mail was numbered 1 Up / 2 Down and the Howrah - Bombay Mail as 2 Up / 1 Down to avoid confusion.
Interestingly, on the Ring Railway in New Delhi, the clockwise direction is marked "up" in the timetables.

in general terms:
East to west is UP
NE to North/South is UP
South to West/North is UP
First number is UP
Second...
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number is also UP when the designation/originating point falls in UP side.
Some time a DN train becomes UP at destination due to geographical position of destination.
So On ....