It is replacing the number by the whole number such that the difference between the number and that whole number is the smallest that it can be.
an integer is a whole number
0.3 is not a whole number and 3.0 is a whole number
There is no difference.
There are no differences.
Yes, it is.
The difference is that all whole numbers are decimal numbers, but not all decimal numbers are whole numbers. For example a whole number such as 1 is a decimal number but a decimal number such as 1.5 is not a whole number.
A natural number is a number which is positive and a whole number. whole numbers can be also negative.