Electrical contractor/electrician help?

The largest wires give electricity to the metal rails of the panel. If they p through a big breaker (usually 100, 150 or 200 amps) to get to the rails, it's a main breaker panel. If the large wires go right to the rails with no breaker it a MLO panel. A main breaker panel is appropriate for 95% of the installations out there. You should consult your state's electrical code; it's most likely NFPA 70, 2008 edition. Don't forget your permit. Work safe.