Up until now the only option for declaring a variable was by using the statement var which is scoped to the nearest function block. let on the other hand is scoped to the nearest enclosing block.
Although Douglas Crockford sees no issue with using var he notes that to developers coming from different language back grounds singling out function blocks is unusual. Therefor he proposes that we use let instead of var. However, the most important point is that pick one of the two and stick with it.
The const statement creates a read-only constant.