A Uniform Resource Identifier(URI) is a sequence of characters that provides a simple and extensible way to identify abstract or physical resources.
A Uniform Resource Locator (URL) is a URI with the location of the resource included. URLs for internet resources include the protocol, domain name, and path to the file on the server.
A Domain Name is a unique label for an entity of the internet. The Domain Name Systemorganizes the internet by identifying the address and type of entity. It also converts domain names into IP addresses that can be understood by machines comprising the Internet.
Domain names have at least 2 parts separated by a period. The right-most part of a domain name is called the top-level domain (TLD). Anything before the period is considered a subdomain of what comes after the period.
Subdomains can be used to house separate websites at the same domain. Google's gmail service is an example of a subdomain of google.com. Google has many different services and websites dedicated to each service and they are all located at the same domain as google's search engine.
Many free web hosing services will let you host your website on their domain using a subdomain. For example if you sign for for a free wordpress blog without upgrading to thier paid services, the url for your blog will be https://yourblogname.wordpress.com.
WWW is also a subdomain but it's a special subdomain that is not completely necessary but still commonly used. It was originally used to indicate that a the domain name is for a web page and not something else like email. One reason to use www today is to separate cookies from a main site from it's subdomains and be better optimized for search engines.
TLDs can be used to classify domains and indicate what type of website the domain name is poitning to.They can be generic like .com and .net or they can be country codes like .fr or .zw. The organization currently responsible for country code top-level domains (ccTLDs) and generic top-level domains (gTLDs) is IANA, which is a function of ICANN..
Seven generic TLDs predate the creation of ICANN in 1998. They are: .com, .org, .net, .int, .edu, .gov, and .mil.