METAR stands for Meteorological Aerodrome Report. It is a standardized aviation routine weather report usually pertaining to a specific airport.
No, METAR winds are provided with reference to true North.
Traditionally official magnetic winds are only given over radio or telephone, while true is used for all other purposes.
No, METARs are observations of existing weather conditions. Refer to TAFs (Terminal Area Forecasts) for expected future aviation weather conditions at a specific location.
METAR altitudes for cloud heights are provided in AGL (above ground level). It is possible MSL will be specified for other remarks.
Typically they are updated hourly or when the weather changes significantly (a SPECI). Some airports are updated every 30 minutes, every 10 minutes or some automated systems issue METARS even more frequently.
METARs is considered valid until a new METAR is issued for the same location. A METAR that is an hour old or more is probably only good for getting a general idea of the weather since the weather can change significantly between reports. It is best to check the weather closer to the time you will actually be at the airfield. That being said if a non-Automated METAR is nearly an hour old and a new one has not been issued, it is likely that the weather hasn't changed significantly enough to warrant the issuance of a new METAR and the latest METAR could still be relatively accurate.
The $ on the end of a METAR is called the Quality Control or QC flag. It means the weather station needs to be serviced and could be producing inaccurate data.
Yes see our METAR and TAF Decode Key Post.