LaTeX is a document preparation system. LaTeX can be used to produce high quality documents, very strong support for Maths and Science.
Overleaf is a website that allows you to create LaTeX documents on the website and save in the cloud. You can also collaborate with others on documents.
LaTeX is pretty comprehensive and takes up a large amount of disk space, so Overleaf gets round this as it has all the components you need.
Tutorials for both LaTeX and Overleaf can be found here.
1 https://www.latex-project.org/ – Main LaTeX project website.
You can also chat to other users of LaTeX on IRC (Internet Relay Chat) on freenode #latex.