As of March 2018, the 457 Visa was replaced with the TSS Visa. It is similar in many ways to the 457, being a sponsored temporary skilled visa, but has both a short term and a long-term stream which determines how long you can stay and whether you are eligible for Permanent Residency down the track.
How does the TSS visa work?
With a TSS visa you can work in Australia for two to four years. You can bring your family with you (they can work and study) and you can travel in and out of Australia as often as you like.
Your prospective employer/sponsor must apply to be a sponsor to recruit overseas workers; nominate the positions they want to fill; recruit the overseas workers to fill the nominated positions; and act as a sponsor for employees applying for a visa.
You as the employee accept the offer of employment from the employer; apply for the visa; and meet all the conditions of your visa.
How do I know how long I can stay for?