Campden Tablets???

I'v done a search on theis section but I'm still confused so thought I'd just ask anyways...

I'v just started making my own wine and got a copy of CJJ Berry's First Steps in Wine Making and it mentions campden tablets but I'm just unsure of when to use them and how many?

Any help greatfully recieved.

Thanks Riala,that's also answered my second question on where to get it......thanks for the link.

Any chemist will sell the stuff used for sterilising babies bottles, can't recall the brand name, but I used them when mine were babies

That'll be Milton 

Cor, fancy that, sterilizing baby bottles with poetry!

I'm fairly new to wine making myself and also using CJJB's book.

His blackberry wine recipe calls for a campden tablet and pectic enzyme to be added at the beginning, 24hrs before yeast is added.

My understanding of this is to sterilise and kill any natural yeasts in the fruit/veg prior to adding a wine yeast.