Difference between Functions CAST() and CONVERT() ?

Both these functions are used to convert values from one DataType to another
But there are some differences between them 
1 CAST is ANSI standard and CONVERT is specific to SQL Server
2 CAST can't be used for formating purposes.
But CONVERT can be used for formating purposes particularly on datetime and money datatype  
Example :
declare @myDate datetime
set @myDate='20121209 12:22:45'
select convert(varchar(10),@myDate,108)
declare @mny money
set @mny =123456
select convert(varchar(10),@mny ,1)
Result : 
----------
12:22:45
----------
123,456.00

Note : 

Also you should be very careful in using the both when you convert values.
Consider the following example

declare @i int

set @i=123456

select convert(char(5),@i),cast(@i as char(5))

The result is

1.----- -----

2.*     *   

It is because the size of the variable is not enough to hold the number In this case you will not get any error.
You should in advance specify the enough length