An inspector checks the safety of your potential new home. Home Inspectors focus especially on the structure, construction, and mechanical systems of the house and will make you aware of only repairs that are needed.
The Inspector does not evaluate whether or not you're getting good value for your money. Generally, an inspector checks (and gives prices for repairs on): the electrical system, plumbing and waste disposal, the water heater, insulation and ventilation, the HVAC system, water source and quality, the potential presence of pests, the foundation, doors, windows, ceilings, walls, floors, and roof. Be sure to hire a home inspector that is qualified and experienced.
It's a good idea to have an inspection before you sign a written offer since, once the deal is closed, you've bought the house "as is". You may want to include an inspection clause in the offer when negotiating for a home. An inspection clause gives you an 'out' on buying the house if serious problems are found, or gives you the ability to renegotiate the purchase price if repairs are needed. An inspection clause can also specify that the seller must fix the problem(s) before you purchase the house.