Mimeo, Technical report, Manuscript: what kind of creatures are they?
Most often I find that these are works in progress, unfinished or otherwise unsuitable for regular academic publication. I've written technical reports as an undergrad as part of an internal project for a non-research entity that simply described the progress we made on a project. There was little context for publication outside the organization itself, and not enough novelty to try to pull some context together. However, these documents were referred to internally to justify further work in the area due to the feasibility that we showed for the project. Sometimes, when they're not too proprietary, some organizations will put these kinds of reports up on their website to show the kinds of things they are working on. Maybe several will be combined at a later point as the foundational parts of a proposal or a "real" paper. 

I haven't seen "mimeo" used ever in my life except in the context of the output of a Mimeograph machine which was an early way to duplicate documents. It's possible that mimeos of technical reports in the scientific report context used to get passed around the big national and industrial labs so that people could find out what was going on internally well before there'd be an internal website or wiki to browse for like-minded researchers. Otherwise, that's not really a common term that I encounter.