malloc vs Paritions

[Kiran C]

Hi,
How are memory partitions different from malloc?
When should memory paritions be used  and when should malloc be used.
What are the advantages and disadvantages of using  one of them?
I have heard the in embedded development all memory is preallocated...
why is this so?
Any such preallocation will be a wastage of system resource,
take for example I need to process some user supplied data whose size is un
known to me
at compile time but known to me at run time....
The which is a better option ...malloc the required size 'or'
use a memory parition of with the worst case size.
Thanks,
Kiran