"chan," usually used in the plural as "chans," can refer to the large array of English-language websites that were inspired by the japanese imageboard 2channel. The largest and most influential of the "chans" is the popular website 4chan, although there are many other similar websites such as 7chan, 420chan, wakachan, iichan, not4chan, gurochan, fchan, sidechan, renchan, and so on and so on.
Their characteristic features are anonymous posting, content being quickly deleted once it becomes old, "sage"-ing, and an idiosyncratic and strange culture.
"Yeah, I browse the chans a lot...4chan mostly, but gurochan when I need some fapping material."
The term "mecha musume" was first used for a series of figurines produced by the japanese company Konami. The figures personified old military vehicles, such as tanks and planes, as cute girls. It's not used to refer to a general art style in which the features of historical military warcraft, or more fanciful science fiction mecha designs. The term means, literally, "mecha girls."
"I love this new mecha musume show. Badass military hardware? Uber-moe cute girls? Why not get both at the same time..."