use of Threadstart delegate

Please resolve this,

For thread initialization what is the difference of using "Threadstart" and without using "Threadstart".

1. Thread worker = new Thread(new ThreadStart(DoTimeConsumingWork));
worker.Start();

2. Thread worker = new Thread(DoTimeConsumingWork);
worker.Start();

Both are working.
What is the difference between two.

Regards
krrish