Latitude and longitude are the two numbers used to describe any location on Earth. Together, they form a coordinate pair that uniquely identifies a point on the globe.
Latitude
Latitude measures how far north or south a location is from the equator. The equator is 0 degrees. The North Pole is 90 degrees N (or +90). The South Pole is 90 degrees S (or -90). Canada spans roughly from 42 degrees N (Windsor, Ontario) to 83 degrees N (northern Ellesmere Island).
Longitude
Longitude measures how far east or west a location is from the Prime Meridian, an imaginary line running through Greenwich, England. The Prime Meridian is 0 degrees. Longitude ranges from -180 (west) to +180 (east). Canada spans roughly from -52 W (Newfoundland) to -141 W (Yukon/Alaska border).
Coordinates are written as (latitude, longitude). Ottawa, Ontario is approximately (45.42, -75.69). The positive latitude means north of the equator. The negative longitude means west of the Prime Meridian. In Canada, longitude is always negative.
Decimal Degrees vs DMS
Coordinates can be expressed as decimal degrees (45.4215 N, 75.6972 W) or in degrees, minutes, seconds (45 deg 25 min 17 sec N). Decimal degrees are easier to work with in software.
Precision
One decimal degree is about 11 km. Four decimal places is about 11 metres. Six decimal places is about 11 centimetres. For most mapping purposes, 4 to 6 decimal places is sufficient.
Learn how the round Earth gets flattened into maps in Map Projections.