What is a breadboard?

A breadboard is used to build and test circuits quickly before finalizing any circuit design. The breadboard has many holes into which circuit components like ICs and resistors can be inserted.

The bread board has strips of metal which run underneath the board and connect the holes on the top of the board. The metal strips are laid out as shown below. Note that the top and bottom rows of holes are connected horizontally while the remaining holes areconnected vertically

Actually, breadboard has got its name, only due to mere resemblance of a bread, structurally. In a bread, we find the bread particles, separated by air molecules, which forms holes in between. Likewise, in a breadboard, we find there are plastic rows and columns, separated by holes, for making electrical connection.
Too simple way of putting it... isn't it?

And regarding the applications, they are used as a beginner's tool for building simple circuits, without much strain and without making permanent connections like soldering, etc. The components can be easily attached and detached from the circuit as and when required.