I have a carton and a jar of Passata (Sieved tomatoes) in the cupboard. Can I use this as a tomato sauce to make a pizza ?
Or is the sauce on a pizza something different in some way ?

Lots of pizza recipes recommend passata.
Some are very runny though, almost like tomato juice.
I'd use it if it's reasonably thick.

I'd cook it first. Gently fry a crushed clove of garlic in some olive oil in a pan over a medium heat,

Add the passata and let it reduce and thicken for about twenty minutes.

Season to taste with salt, black pepper and perhaps a little sugar to balance it out and round out any acidity.

Let it cool and you have an excellent thick and tasty tomato sauce for spreading on a pizza base.

If it's too thin then you can mix it into some tomato puree.

I agree with this, better to cook it to thicken it than add tomato paste and a little sugar is enough to take the acidic taste away.

I've used passata several times for a pizza calzone topping. I much prefer it to tinned tomatoes, as i hate the texture of tomatoes (but love the flavour). I would use it anywhere recipes call for tomatoes, chilli, bolognese, etc.
If you roast some peppers and garlic and blend with passata it makes a fantastic soup too