METARs are weather reports at an airport. Usually look something like this:
METAR CYAM 131800Z 10008KT 15SM FEW035 FEW140 FEW240 13/04 A2989 RMK
SIGMETs are "significant meteorologic" events ie thunder storms where a weather station will put out to caution pilots about.
Heres an example:
WSCN32 CWEG 131635
SIGMET B1 CANCELLED AT
LLWS HAS BECM MDT
Decoding them is another thing. A Google search should probably explain that.