Each computer connected to the Internet must have a unique number called the IP number; No two computers can have a same public IP number.
In order to avoid the need to memorize numbers, domain names were created. Special computers spreaded over the Internet provide an information service that you can use to find out what is the IP for a given domain. Those computers are called Domain Name Servers (DNS).
Domain names are controlled by companies "Domain Registrars". Usually there are no guarantees that the domain name belongs to the company that you think.

URL

Universal Resource Locator - URL is a combination of a domain name with a directory structure. It allows you to make a reference to any resource (web page, picture, sound wave, etc.) on the internet.

Certificates and Secure Sites

In order to help you trust that you are reaching the institution that you really want, some web sites acquire a site certificate. The site certificate ensures that someone has verified that there is a legitimate company that owns that web site. Often times, sites with certificates must be accessed through a secure protocol (HTTPS) and a warning will pop up if something is suspected wrong with the certificate.
Secure Protocol (HTTPS) establishes a connection that only exchanges encrypted data between you and the remote server. This is to avoid malicious attempts to find out your passwords or other sensitive information about you.

Phishing

You may often receive email that directs you to a web site. Be careful that you may, in fact be giving away your information to the wrong web site.
Avoid entering information when you don't trust the web site; also be careful that con artists have deceiving domain names. For example there is no guarantee that www.grades-franca.com belongs to www.franca.com
When entering sensitive information such as your social security number and credit card numbers, make sure that you are using secure connection (there is usually a padlock icon indicating that).
Finally, if an email requests you to connect to your bank, avoid following the link and prefer to log into the bank web site as you usually do.