Using time.h library instead of delay

I am new to Arduino an are working on a project with a group. I am required to use time.h instead of delay. I am not sure how you change to that, or why. If someone can just explain why it would be benefical to switch to time.h and what exactly it does that would be really helpful.

First step is to read the documentation: http://www.pjrc.com/teensy/td_libs_Time.html

Read these also

How to do multiple things at once ... like cook bacon and eggs http://www.gammon.com.au/blink

Demonstration code for several things at the same time http://forum.arduino.cc/index.php?topic=223286.0