//Create a new account called "Test Account" that we will create a link for later.
Account acc = new Account(Name = 'Test Account');
Insert acc;

//Get the base URL.
String baseURL = URL.getSalesforceBaseUrl().toExternalForm();
System.debug('Base URL: ' + baseURL);       

//Get the URL for the current request.
String currentReqURL = URL.getCurrentRequestUrl().toExternalForm();
System.debug('Current request URL: ' + currentReqURL);        

//Create the account URL from the base URL.
String accURL = URL.getSalesforceBaseUrl().toExternalForm() + '/' + acc.Id;
System.debug('URL of a particular account: ' + accURL); 

//Get some parts of the base URL.
System.debug('Host: ' + URL.getSalesforceBaseUrl().getHost());   
System.debug('Protocol: ' + URL.getSalesforceBaseUrl().getProtocol());

//Get the query string of the current request.
System.debug('Query: ' + URL.getCurrentRequestUrl().getQuery());