Difference between Distribution board main switch and mains
Distribution box (aka distribution board, breaker panel, electric panel, breakerboard) is the the enclosure where the main power feed comes in and is distributed to individual circuits
Main switch is the master switch that shuts off all power, and is the first thing the power runs to that comes into the panel.
"Mains" (aka mains voltage) refers to main line-level voltage (generally 120V or 240V, depending on where you live) and is used to refer to things connected to the electrical system and thus subject to code. It's often used to distinguish from something that is "low voltage" 1 (eg, 12 or 24V circuits)





1 Note, technically this is called "extra-low voltage", and is anything &lt;50V AC or &lt;120V DC. "High voltage" is anything >1000V AC, and "low voltage" is everything in between, but really only linesmen that work for the power company call it that. In a house or commercial situation, you basically never see anything higher than 240V so we just use the terms "mains" or "low voltage".