TAF is a format for reporting weather forecast information, particularly as it relates to aviation. "TAF" is an acronym of 'Terminal Aerodrome Forecast'.
Generally a 12 or 24 hour forecast, it complements and uses similar encoding to METAR reports.
TAFs are always produced by a human forecaster based on the ground at a particular station. For this reason they do not have the same coverage as METARs but they are also very much more accurate than Numerical Weather Forecasts, taking into account local, small-scale, geographic effects.