A common question is what exactly is meant when controllers and pilots use the term "heavy." I've discussed wake turbulence previously, and perhaps will again, but for today I'll limit myself to the FAA's definition of a heavy jet.* The FAA (and ICAO) define a heavy as an aircraft that has a Maximum Takeoff Weight (MTOW) of at least 300,000 pounds (136 tonnes). This is higher than when I first talked about wake turbulence; when I first started out as a controller, a heavy jet was one that could weight 255,000 pounds (115 tonnes) for takeoff. The aircraft weight categories were adjusted a few years ago to bring the US standards more in line with what was happening in the rest of the world.
A simple rule of thumb is that if an airplane is a wide body, it's a heavy. There are some variants of the B-707 and DC-8 that qualify as heavies, but these are pretty rare nowadays.** It is worth noting that an aircraft is classified as a heavy if it is capable of weighing 300,000 pounds at takeoff; the term does not reflect what the aircraft actually weighs at any given moment. We see this sometimes when a B-747 gets airborne just a few thousand feet down the runway when it more commonly requires most of the runway to get into the air.
When talking to or about a heavy jet, air traffic controllers are required to append the term "heavy" to the callsign or aircraft type in radio communications. This serves as an alert to other pilots in the area who need to be aware of the possible wake turbulence encounter. While many of the pilots of heavy jets will refer to their aircraft as a heavy, they are not required to do so; it's an ATC rule.
* - Nearly all of the heavy aircraft are jets. The Russian Tu-114 is a four-engined turboprop airliner that has a MTOW of over 360,000 pounds (164,000 kg). This aircraft is better known to the west in its military form, the Tu-95 "Bear" long-range bomber. A new prop-driven addition to the list of heavy aircraft is the Airbus A400M Atlas. This is a four-engined military transport turboprop with a MTOW of 310,000 pounds (141,000 kg).
** - As LAX is a civilian airport, served almost exclusively by commercial airlines, I'm not including military aircraft in this discussion. Some military heavies include the B-1, B-2, and B-52 bombers, along with the C-141, C-17, and C-5 transports. Also the B-707's military cousins, the C-135, KC-135, and E-3.