Who can give me the answer to this question please.

Both.
It is like a poison to the body, which is why you feel like vomiting after a hard session.
But this in turn forces your body to make adaptations, thus increasing your training threshold.
I could go into detail but this ain't a university.

Edit: and I can't be bothered right now, but I'm sure google can elaborate on both sides of the argument.

hang on hang on edit again to elaborate:
Lactic acid is produced by your body to help create energy, it's created by chemical reactions in your muscles. Eventually, The amount of acid in your muscles becomes so great it causes that burning sensation (actually caused by the chemical reactions, not the acid itself) and your limbs turn to jelly, until you recover.

erm, I hope that made some sort of sense.

Is it some sort of performance battery? ?

Lactic acid is powerful organic acid and its accumulation can result in feelings of exhaustion and muscle fatigue, However, lactic acid also has many benefits .Athletes in training need both high intensity and distance training in order to obtain optimal control on lactic acid. High intensity exercise will develop cardiovascular capacity to maximize oxygen transport to muscles so there is less reliance on carbohydrate breakdown to lactic acid. High intensity exercise will develop cardiovascular capacity to maximize oxygen transport to muscles so there is less reliance on carbohydrates breakdown to lactic acid. Furthermore, this increased circulatory capacity will maximize lactic acid removal from muscle and blood. Distance training is necessary to develop the tissue enzyme adaptations that will maximize use of fatty acids for energy, which will result in less lactic acid formation .

Well there you go, kinda answered your own question.