A top-level domain (also known as an extension or TLD) is used to identify a specific territory or a specific type of business.
In the first case we will talk about ccTLD (country code top-level domain), as, for example, in the case of .it, .fr, .es domains which will identify specific geographical areas, such as Italy, France and Spain.
In the second case we will talk about gTLD (generic top-level domain) usually used by organizations or companies to identify their field of action (for example, the .com extension is often used by commercial organizations).
A second-level domain generally identifies the subject, product, brand, company, or service promoted by the domain.
For example in www.utixo.net it will be identified as a second level domain, representative of the company that promotes its services through the aforementioned domain. The second level domain describes, in summary, the content of the website.
A third level domain (also known as a subdomain) identifies a specific part or section of interest of the domain itself. For example in mail.utixo.net they represent a deepening of a part of the entire domain.