From the site:
Backblaze is unlimited so you never have to worry about going over some limit and paying extra. Unlimited storage. Unlimited bandwidth. Unlimited file size. And we’re committed to unlimited online backup for the long-term.
We designed Backblaze to be really easy to use. There is no need to select the folders and filetypes you want to backup. That means all your data will be backed up without having to do anything. The service backs up everything on your computer except your operating system, applications, and temporary files.
Whether directly connected or over WiFi, If you are connected to the Internet, you are backing up.