ZH 
EN 
Winter batteries hold much less charge than in mild weather. Leave the lights on, or park for two weeks, and the battery goes flat and the engine will not start.
By pushing (bump start)
A familiar emergency method, but a last resort. It only works with a manual gearbox and it puts wear on the engine and clutch. Have help to push. Check the road first: a downhill slope is easiest, flat is workable, and uphill you should turn around and push downhill. Safety first.
With jump leads
The better option. Positive to positive, negative to a good earth on the flat car, start the donor, then start yours. Keep the clamps apart.
With a jump pack
A portable pack starts the car with no second vehicle. Keep one in the boot in winter.
Avoiding it
Test the battery yearly after three years, keep terminals clean and tight, switch off all loads before leaving, and take an occasional longer drive to recharge.