Street furniture is any fixture (generally installed, paid for and maintained by some level of government, though not always) that's designed to be interacted with by the public at large. Bus shelters, park benches, pedestrian traffic flow regulators and mailboxes are all examples of street furniture. They, along with things like signage conventions and architecture help establish a region's character in such a way that they're almost universally ignored unless one happens to be using one at the time.
It's important to note that, by and large, the purpose of street furniture is intellectually passive - a stop sign wouldn't qualify because it's informative. But (and it's a big but) advertising doesn't count towards the definition - a bus shelter is considered to be street furniture no matter how blatant the ads are.
From an advertising perspective, street furniture is a relatively minor part of a brand strategy for low-density residential areas and a costly logistical nightmare for urban areas with heavily used public transportation systems or foot traffic - an ad in a metropolitan subway tunnel stays active for a maximum of sixty days before it's rotated; the bigger the system, the more complicated the rotation gets.