Shodan https://www.shodan.io/ is a search engine that lets the user find specific types of computers (routers, servers, etc.) connected to the internet using a variety of filters. Some have also described it as a search engine of service banners, which are meta-data the server sends back to the client. This can be information about the server software, what options the service supports, a welcome message or anything else that the client can find out before interacting with the server.
Shodan collects data mostly on web servers (HTTP, port 80), as well as FTP (port 21), SSH (port 22) Telnet (port 23), SNMP (port 161), SIP (port 5060), and Real Time Streaming Protocol (RTSP, port 554). The latter can be used to access webcams and their video stream.