Humble Bundle started out as occasional bundles of indie games. Now they typically have a new game bundle every Tuesday, a new book bundle every Wednesday, and the occasional mobile (Android) bundle on a Monday. Bundles typically run for two weeks, so there'll usually be two game or book bundles active at a time.
Big publishers have gotten in on the act, so it's not just indie games any more. But the big publisher bundles will usually be keys only (Steam, Uplay, or Origin). The indie bundles are more likely to have a Steam key and a DRM-free download. Sometimes you get soundtracks.
There's been one or two Nintendo bundles and a Playstation bundle. You get 3DS/WiiU or PSN keys. No Xbox bundle yet.
The Monthly subscription consists of a "headline" games that they advertise and that you get immediately upon subscribing (this month it's XCOM 2) and five or six mystery games that you only discover once the next monthly bundle starts.
There's also a Humble Store. The 10% discount you get with the Monthly subscription is good there. Like Steam, they sell games and publisher bundles, and have regular sales. Unlike Steam, some games at the Humble Store also give you DRM-free copies. (So, they're like a hybrid of Steam and gog.)
Just to be clear, the weekly bundles, the monthly bundle, and the store are separate things that Humble does. The unifying theme is that part of your purchase goes to charity. You can specify how much in the weekly bundles. It's fixed in the monthly. The store gives you the option of saving (Scrooging?
) the 5% of your purchase that goes to charity for yourself.