A breadboard is a grid of sockets for developing electronic circuits and microcontroller projects, composed of conductive strips beneath the nonconductive cover. Because it is solderless, it's useful in prototyping, but more susceptible to falling apart when handled roughly.

The main grid, which is 10 columns wide and labelled a through j, has connections across every row, connecting a through e of every row, and f through j of every row. On most boards, the 4 columns bracketing the main grid are all connected across every column, from the top of the board all the way to the bottom. These 4 columns are traditionally used as power and ground lines.

As a caveat, some large boards held in the physical computing lab have the 4 external columns split in half (observe diagram below) and are therefore not conductive all the way down.

The large breadboard in particular is used when you need to make a lot of connections. Maybe you have a lot of parts with complicated chip wiring set-ups, or you are constructing a project where it's imperative to have all the wiring in a central location for personal legibility, as opposed to in several smaller breadboards dispersed throughout. Either way, the large breadboard is a great tool for prototyping many solderless connections.