WiFi access point - device which share access to the Internet via radio channel. Typically access points coverage are limited by small territory. It's a law-limitations. Howewer, in large areas (like airport terminals, train-stations or big hotels) can be deployed huge WiFi network. In this case many hotspots consolidated in same network, uses one authorisation credentials and always allows "roaming" between access points which owned by one company. Common access point looks like an inconspicuous rectangle with several antennas (or without it). Frequently access points fully no visible (for example, mounted behind false ceiling).
Not always but very often. Today free wifi access has become almost mandatory for service businesses, catering, entertainment companies etc. Access to the web on their territory are fully charge-free, accessible for all without exceptions or only for clients (in this case, access-password may be printed in a carte). In case of huge networks, which can cover city area, you can get internet access after see short advertising video. The need to view the advertisement can be repeated after a while.
It is very simple. Just turn on WiFi adapter of your mobile device (like notebook, tablet or smartphone) and find network name. This name usually associated with company name. After that tap on this name or push "Connect" button. If network available for all, your device connect immediately. If access available for customers only, fill password in pop-up window and press "Connect" again.
In most cases you dont need to do something special for surfing websites if connection established, just open your web-browser and go. But sometimes, too rarely, authorisation page pass can be required. System can claim your client number, invoice number, number of your mobile phone or solve simple captcha. Obvious press on "Yes, I will go to the Internet" button are possible too.
Not always. Any free ways to get the Internet involve access-to-all by definition. So, any client of hotspot (or even point's administrator) could be malefactor. As a result, you shouldn't use any confidential information when you stay connected in free WiFi. Often happens a men drank a cup of coffee in the airport cafe (at this time he looked facebook ribbon), when his plane landed, his facebook account was hijacked. And although similar stories are rare, be warning when you use a public hotspot. Especially, do not use your bank account at this time. If using of "sensitive data" in public WiFi is necessary, is not superfluous to use traffic encription.