Great question. We define bandwidth as “bytes sent to your visitors’ browser, or bytes sent to remote services that you proxy to” (details on proxying: Redirects and rewrites | Netlify Docs).
Bandwidth sent to browsers is pretty straightforward, so I won’t explain it further unless you need clarification, but here’s where the caching our CDN does helps you save money - we don’t re-send unchanged content when a visitor refreshes the page. More details on how we handle that here: Better Living Through Caching
Bytes used for proxying is a bit less obvious. Our various network providers charge us for outgoing bandwidth - and in case you proxy to your API, this will be what the browser sends. If your visitors upload a 10MByte PDF - then you used 10MBytes of bandwidth during that upload, despite their browser probably receiving only a few bytes (e.g. HTTP 200 OK). This is a less likely use case since most people don’t use proxying.
Build & deploy are not covered by bandwidth. Certainly building when you don’t need it is wasteful, so we appreciate efficiency if you can manage it. But we don’t charge per build right now and it won’t affect bandwidth calculations.
Edited Oct 2019 to add:
We do now charge for build minutes, so you’ll want to make sure you aren’t building needlessly. Each plan comes with a free allowance and this article may help you optimize your build minutes usage now: [Support Guide] How can I optimize my Netlify build time?