If you hold information in hard-copy or electronic form about employees, customers, potential customs or suppliers, you muse only use it for the purpose for which it was gathered.
The individual must have given consent or have been informed you have the data and know what you want to do with it.
It must be accurate and up to date and help for no longer than necessary. The data must be kept securely.
You must provide a copy of data you hold if the individual requests it. You must consider amending any inaccuracies they report to you.
You have to register with the Information Commissioner if you are using the data you hold for more than the day-to-day operation of your business.