The Long Count is part of the
Mayan Calendar notation. It is determined by the number of days that have passed since the beginning of the current
Mayan era, sometime in September, 3114 BC, although there is some
debate on the actual date.
There are five units in the long count: Bak'tun, K'atun, Tun, Winal, and K'in. They are as follows:
1 Bak'tun = 20 K'atun = 144,000 days
1 K'atun = 20 Tun = 7,200 days
1 Tun = 18 Winal = 360 days
1 Winal = 20 K'in = 20 days
1 K'in = 1 day
For reference, today would be labeled as 12 Bak'tun, 19 K'atun, 9 Tun, 12 Winal, 1 K'in in Long Count (using 584,285 correlation) by the Mayans. In modern notation, these numbers are given as a string of numbers, which looks like 12.19.9.12.1.
Sources:
"The Mayan Calendar", http://www.halfmoon.org/calfork.html
"Gregorian-Mayan Date", http://www.pauahtun.org/cgi-bin/gregmaya.py