The higher maths learn a few extra topics, and also more challenging questions from the standard stuff.
The exam is like this, the first 4 questions (1-4) are common to maths and higher maths. The last two questions (5 & 6) are different for normal maths and higher maths, the higher maths Q5 & Q6 contain extension topics as well as harder applications of the standard topics, and for normal maths Q5 and Q6 are just similar to Q1-4. That's the only difference between higher & normal maths.
If you're eligible to do higher maths then I highly recommend you take it up, you get scaled up for it, just like you get scaled up for doing 4U in the HSC (maybe not quite as much, but higher is also more interesting/challenging).