A URL Template is a way to specify a URL that includes parameters that must be substituted before the URL is resolved.
It's a part of URL containing variable names enclosed in braces e.g. www.example.com/user/{id}.
The URL templates are defined on the server side. Their values must be substituted by the underlying server software (the template processor) before processing.
Client requesting the URL is unaware of such templates, that is it sends the request normally: www.example.com/user/234. On server side, the template {id} in www.example.com/user/{id}, is matched with the last part of the requested URL and value for id is substituted to 234 before it can process the request.
