There's really no difference between a spatula and a fish slice although the fish slice is a kitchen utensil with a wide, flat blade with long holes in it, used for lifting and turning food while cooking.
A spatula is also the same thing as a fish slice except the spatula may or may not contain holes.
A spatula with holes is sometimes used for mixing and beating ingredients and to prevent you from overbeating your ingredients resulting in better cakes and sauces.
Some spatulas also have holes to allow grease and oils to drain from the food as you're picking the food up with the spatula.
The reason a spatula has holes in them is for grease and other liquid to drain out away from the food when you pick it up.
For example if you're cooking a hamburger and it's greasy the holes in the spatula would allow grease to drip off and out away from the hamburger making it less greasy.
You can also buy spatulas without the holes but the spatulas with the holes have a purpose as in letting the liquid or grease or oils drain away from the food.
Spatulas get their name because of the act of turning the food over with them.
A spatula with holes is sometimes also called a slotted turner.
A spatula without holes is called a solid turner.