METAR is a format for reporting the latest available weather information. It is similar to but distinct from TAF.

METAR weather reporting format is used by aviation weather stations for routine weather reporting (typically hourly). The information contained in a METAR report includes the location of the weather station, date & time of the weather observation, the wind speed & direction, visibility, precipitation, cloud cover, temperature, and barometric pressure.

A similar, but distinct, type of weather information report is TAF, or Terminal Area Forecast, which provides a forecast rather than the most recently available information.

A typical US METAR observation looks like this:

METAR KTTN 051853Z 04011KT 4SM -SN BKN050 OVC100 02/M02 A3006 RMK AO2

This is a routine hourly report (METAR) at Trenton-Mercer Airport (KTTN).
The observation was taken on the 5th of this month, at 1853 UTC (051853Z).
The winds were from 040°, at 11 knots (04011KT).
Ground-level visibility was 4 statute miles (4SM), with light snow falling (-SN).
There was a broken cloud layer at 5000 feet (BKN040, and an overcast layer at 10000 feet (OVC100).
The temperature was 2° Celsius, and the dew point was -2° Celsius (02/M02).
The barometric pressure (altimeter setting) was 30.06 inches of mercury (A3006).
There are remarks (comments, followingRMK), which in this case specify the type of weather station equipment that generated the report.

Detailed information on METARs, including a reference for decoding the information, can be found on Wikipedia, as well as through a variety of other sources.