Hi i am looking for a case for my atx motherboard and some cases say that it with fit standard atx boards. my question is is there a diference between "Standard ATX" and "ATX" Or are they the same thing?
No, there's no difference. Standard ATX is same as ATX. Sometimes it's referred to as Standard ATX simply to make sure there won't be any confusion, as ATX by itself could refer to any ATX form factor (mATX, WATX,...)
The only differences in board sizes are generally seen between ATX and the smaller Micro ATX boards. With the exception of mini tower or compact cases mid and full tower cases will readily accept either type being the standard format. Most cases also allow a degree of flexibility for positioning the board mounts.
If you get into a custom mini case like some seen now with only two 5.25" drive bays or some other mini tower case then you have to prepare for a micro not full sized board for those. Otherwise the term standard means it will accept all atx format boards whether full or micro. BTX is a different format there however.