The n-th braid group consist of all different ways n strings can be braided. The generators are all the over-crossings and under-crossings of neighbouring strings. They can be also imagined as world lines of n particles in 2 dimensions which changes its order.
Examples
The generators
and
:

And the transformation like:

is not a braid.
Applications
The braid group plays key-role in the topological computing.

