A top-level domain (TLD) is the last part of an URL, such as .com, .org, or .net. It's also known as a domain extension. The TLD Portfoliobox can register are:
Regardless of the TLD, all domain names must follow the same character rules. You can use letters (abc), numbers (123) and dashes/hyphens (---). Spaces are not allowed and the domain can't begin or end with a dash.
If you're interested in a TLD that isn't included in the list above, such as country specific ones, you can register one with a third-party provider and configure it.