The columns are as follows:
ICAO - this is a 6 digit hexadecimal code that is assigned to the airframe by the ICAO. It is unique to the aircraft and doesn't change throughout its lifetime, unlike the registration number.
Flight - this is the flight number or callsign.
Squawk - the mode A transponder code that is set by the pilot according to instructions from ATC. It is used to identify aircraft on radar screens. It can also indicate problems (7700 is emergency, 7600 is radio failure, 7500 is hijack)
Altitude & Speed - I think these are barometric altitude and GPS speed (how fast the aircraft is moving over the ground), rather than indicated or true air speed.
Track - the direction the aircraft is moving over the ground.
Msgs - the number of messages received from the aircraft.
Seen - the time since the last message received from that aircraft. 0 indicates a message was just received, and for example 40 means that a message was last received 40 seconds ago.
The mutability fork renames the last column to age which is a bit more intuitive, and also adds a range column.
The lines in green are aircraft transmitting full ADS-B data. This includes their position and a whole lot of other data. Lines that are white are aircraft that are only transmitting mode S (and maybe mode A/C if you have that option enabled in dump1090). Mode S does not include position, speed or track data, so those columns remain blank.
You can read more about the various transponder modes and what they do here:
http://en.wikipedia.org/wiki/Secondary_surveillance_radar
