The start of the word is the most important the bit that s to do with the users. It is the user s experience of trying to do something with a site that is the critical thing to get right, and this can only be done effectively by testing with real users. Sometimes, a video link or two-way mirror allows other staff to watch without disturbing the test. The users are reasonably representative of the target audience for the site. If there are different target audiences, each must be tested separately.
Testing can take place at almost any stage of development in general, the earlier the better.
Simple tests of early prototypes allow problems to be fixed, and then the next iteration of the site can be tested. This is a much more efficient use of resources than completing the site and doing one big test at the end. But only users can really tell you whether your site meets their requirements. However experienced the usability consultant who advised you on your site, there will always be some aspects that real users will find problematic.