There are a few answers for this one and I see lots of it living here in San Antonio, Texas. First, your foundation could be moving or settling a bit. Don’t be alarmed. Most foundations will settle over time and a crack in the drywall isn’t necessarily a big deal. Simple taping and floating can hide the cracks. Depending of the the size and location of the crack, you may have to texture and paint the areas that are repaired. Your local home center will have cans of spray texture for about $8.00. I’ve seen some do it yourselfers do a good job and I’ve them do horrible jobs. If the cracks are forming in areas such as a living room or kitchen, I would recommend hiring a professional. But, it the areas are in bedroom or less inconspicuous places, you can probably get away with doing it your self. Other problems could be: framing is off or separating or installed improperly, joist or rafters are separating or installed improperly, etc. A good contractor can help determine the cause and a solution to fix the problem.
Now, if you see cracks in the floor tile or major cracks in your drywall, read on:
You can check your foundation yourself by getting a level and placings it on the floor in different areas throughout the house. If the level is off, then it is obvious that your foundation needs attention. A foundation leveling company can help determine what the problem is and how to fix it. Have them show you how and where the house is settling. Ask what equipment they used to determine their findings. I recommend getting at least 3 competitive bids.
If you are considering a home remodel or room addition, your local contractor can repair the cracks while his drywall crew is working on the new construction.