Downloading apps on my android phone through wifi!!?

I know downloading things on my android phone (samsung galaxy s3 mini) takes up loads of data, but if I only download whenever I'm connected to wifi, will it be free and not charge me on my data plan contract?

Thanks.
Update: Also if I want to use uTorrent to download torrents then over wifi it will still be free right? Even if its using lots of GB's up? Cos I only have 500mb contract.
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