To know what a grant writing is, you must know what grants are. Grants are funds that are provided by an institution (i.e.; grantors), to a person, program or organization that the receivers do not have to give back- like a loan will require you to do.  

Grant writing is the process of crafting a written proposal to receive grant funding from a grant making institution in order to fund a program or project.

There is always a demand for grant writers, with the common employers relying on them for financial support, like non-profit organizations. The leadership role a grant writer takes is the research by securing the necessary funds.