It depends on what you are doing, and what you need.
If you are iterating through a collection of items, and do not care about the index values then foreach is more convenient, easier to write and safer: you can't get the number of items wrong.
If you need to process every second item in a collection for example, or process them ion the reverse order, then a for loop is the only practical way.
The biggest differences are that a foreach loop processes an instance of each element in a collection in turn, while a for loop can work with any data and is not restricted to collection elements alone. This means that a for loop can modify a collection - which is illegal and will cause an error in a foreach loop.
For more detail, see
MSDN : foreach[
^] and
MSDN: for[
^]