Just about any "ordinary, necessary, and reasonable" expense that helps you earn business income is deductible. What's ordinary and necessary? The IRS has defined this as anything that's "helpful and appropriate" for your business. For example, buying a computer, or even a sound system, for your office or store can be an ordinary and necessary business expense. Buying the same items for your family room cannot be a business expense, however.
A few things are specifically prohibited by law from being deducted even if the expenses are for the purpose of conducting business -- for instance, a bribe paid to a public official. Other deduction no-nos are traffic tickets, your home telephone line, and clothing you wear on the job, unless it is a required uniform. For more information, see Nolo's article Small Business Tax Deductions.