If the pattern calls for size8 and I use bigger, how will it change or if I use smaller how will it change?

Needle size suggestions are just that..a suggestion. What you need to be concerned about is the gauge. Everyone knits differently. Some people do need to go up or down a needle size depending on whether they knit tightly or loosely and fabric may look identical in the end.

Generally though a large size will give you a looser, more drapey fabric. A smaller needle will make it tighter and denser. Going up or down one size won't make a huge difference, but again it comes down to gauge. If you're making a sweater and use a large size needle it will make the whole thing slightly large. A smaller needle will make it smaller. If you're knitting a scarf or a dishcloth it's not a big deal. Go with what you what makes the fabric you like.

It all depends on your gauge in comparison with the original pattern. Assumming your gauge matched the gauge with the size 8 a bigger needle size would result in a bigger knitted item with the same amount of sts as the original. A smaller needle size would make the item smaller.

Say you are knitting a scarf that the gauge is 20 st x 24 row = 4" x 4" on size 8 needles. If you casted on 30 sts your scarf would be 6" wide.
If you switch to size 9 your gauge might become like 18 st x 21 row, therefore the 30 sts would make your scarf wider so about 6 2/3" wide.
And finally a smaller needle size, say a 7 would make a gauge of 23 st x 26 row, a 30 st scarf would be thinner so about 5 1/4" wide.

This is all assuming your gauge matched the original gauge with the original needle size. A lot of times a person's gauge is different and they will need to go up or down in needles sizes to get the correct gauge.