tldr;
Although both var++ and ++var increment the variable they are applied to, the result returned by var++ is the value of the variable before incrementing, whereas the result returned by ++var is the value of the variable after the increment is applied.
Further Explanation
When ++var or var++ form a complete statement (as in your examples) there is no difference between the two. For example the following
int x = 6;
++x;
assert x == 7;
is identical to
int x = 6;
x++;
assert x == 7;
However, when ++var or var++ are used as part of a larger statement, the two may not be equivalent. For example, the following assertion passes
int x = 6;
assert ++x == 7;
whereas this one fails
int x = 6;
assert x++ == 7;
Although both var++ and ++var increment the variable they are applied to, the result returned by var++ is the value of the variable before incrementing, whereas the result returned by ++var is the value of the variable after the increment is applied.
When used in a for loop, there is no difference between the two because the incrementation of the variable does not form part of a larger statement. It may not appear this way, because there is other code on the same line of the source file. But if you look closely, you’ll see there is a ; immediately before the increment and nothing afterwards, so the increment operator does not form part of a larger statement.