LaTeX is a software system for document preparation, especially used as an editor for scientific documents while it makes easy to typeset complex mathematical formulas.
There are many LaTeX editors freely available on the internet, one of which, Overleaf (used to be called ShareLaTex), is a widely used because it is a collaborative cloud-based version.
A LaTeX editor allows you to create and compile a plain text file - with file extension .tex, - which includes LaTex commands, so that eventually you generate a MS-Word like document.