NewGRF files are files containing sprites and pseudo-sprites that can be used to describe extra vehicles, industries, cargoes etc. for use in OpenTTD and TTDPatch. The most used tool to encode/decode GRF files is GRFCodec (nightly download). GRFCodec can read NFO files and convert them to GRF files (GRF file format). NFO is a low-level language that is tied to the GRF spec. It uses hexadecimal numbers by default in most places and doesn't have any textual identifiers. While the language is very powerful, it's also hard to learn. NML was started mainly to provide a higher level language so coding GRF files will become accessible to more people.
A good place to discuss NewGRFs which are being developed are the tt-forums, especially the NewGRF sub-sections. Questions concerning setup up of NewGRF projects, programming or drawing or just discussing features and presenting work being done are well-placed there. When you are satisfied with a NewGRF and consider it tested enough that it really can be considered to be played with by the general public, you can publish your NewGRF in the NewGRF releases sub-forum and you might also consider to publish it to the Online Content Server of OpenTTD, so that other players can easily download your NewGRF and use it for future games. It will increase the popularity and use of your NewGRF tremendously.