A build is basicly a template, or a way how you plan to assing all you skills, and stats point.
All charecters have a build, since the way you are using you skill up and stats points is your build.
However there are some builds wich are more efective than others.
Often it is better to plan ahead before you starts the game how your charecter will be, rather than just chosing skills, and stats randomly as you level up, because you may end up with a charecter wich is not very efective.
Basicly before you start you character find out how you plan to play it, do you want to use ranged weapons, or short combat, or mostly spells ?
Then you should assign your skills or stats points according to the play style you wish to play.
If you just got the game and plan to go platinium with it i will recomend this..
First play at least 2 or 3 diferent classes to level 15. since there is a trophie for each class reaching level 15, you have to do this sooner or later. So you might as well start with it to get a litle understanding about the game mechanics. By doing that you should get a basic understanding about how to Combat art runes, skills, and stats points are interacting with each other.
In general it is best to only read 1 rune of each combat art, to keep regen timers down.
When chosing skills it is most efective to chose one primary, and maybe a secondary combat art tree a focus on leveling the skills in those. If you spread to much out in all 3 combat arts your charecter may be a litle good in all things, but not really good in any of them.
As your charecter grow in level, and you find out wichs combat arts you use mostly, you can eat a rune once in a while, only if you feel the regen timer is fast.
Also another good way is to try and forge runes into your euqipment, then you can always change euqipment if your timer is to high, or if you think the timers are ok, you can eat a rune.