What are residential proxies?
A residential proxy routes your requests through an IP address tied to a real home internet connection rather than a data centre. Instead of your traffic arriving as an obvious server, it arrives looking like a household in whatever country or city you are targeting, which is why it gets blocked far less often than a datacenter address and why it suits location specific content.
You can run them rotating, meaning a new address on every request, or sticky, meaning the same address for a whole session. The choice is not about quality, it is about whether consecutive requests should look like one visitor or many. Market research, ad verification, search monitoring and collecting public data at scale are the usual jobs.
What to ask a provider
How the addresses are sourced, and whether the people whose connections carry your traffic agreed to it. That question separates networks quickly, and a provider that will not answer it plainly has told you the answer.
Worth knowing
Residential proxies do not make anything unlawful acceptable and they are not an anonymity switch. They are a networking tool, not a shortcut around the law, and any provider implying otherwise is selling you a problem.
Other questions
These answers describe how the NullVault proxy network works in practice, across residential, ISP, mobile, datacenter and IPv6 pools.