Windows can be secured in a number of ways, some more effective than others. The type of window locks you choose depends on the window itself, the physical location and how much you're willing to spend. Door locks are rarely effective for locking windows for a number of reasons that will be covered in more detail below.
Generally speaking, securing windows in houses is more difficult than doing so with doors. To begin with, windows are simply far more numerous, and are generally given less scrutiny by homeowners, which maximizes the chance of an oversight in securing them, however, window security locks are generally cheaper than door locks, and better matched to their task. Windows are - by definition - usually mostly made of glass, and unlike doors, can usually be broken fairly easily. Due to the noise and attention this creates, however, burglars will tend to avoid doing this if they can. Therefore, whenever talking about security, it is worth remembering that window locks alone do not make a window secure. Good lighting, and if possible, high visibility to the street or neighbors are both important factors.
Perhaps the most common type of lock are common "window sash locks". Obviously, these are made for sash windows, which consist of a frame holding the glass suspended by two cords, which run up the channels and over a pulley on either side, with the other end attached to a pair of counter-weights, roughly balanced to the weight of the sash frame and glass. The sash lock has a fixed bracket which is attached to the upper, unmoving part of the window, and catch attached to the sash frame, which pivots to slide into the retaining bracket. They cannot be opened from the outside without breaking the window, and as they are a simple mechanical measure without a key, are relatively cheap. However, if your thief has the luxury of seclusion and the window sash locks are not securely fixed to the frames, or the frame itself is not terribly robust, then there is a chance that your thief will force the lock.
There are alternatives to common window sash locks, and of course, not all windows are suited to this of lock anyway. Deadbolt style window locks are available for windows, which can require a key to open, even if the glass has been broken. These are generally more expensive than simple sash locks, but can provide an extra measure of security.
There is almost no condition under which you would want to use window locks that have keyed entry from both inside and outside the house. If you use a lock that has an external keyhole, you put your belongings at unnecessary risk by allowing the possibility of picking, which is a quiet, and usually quick process.
Child safety window locks are an important consideration if you have windows above ground floor level. Safety window locks are attached to the frame and window itself, and only allow the window to open a certain distance, determined by where you attach the lock. This way you can still open your windows for ventilation, but minimize the risk of an unfortunate mishap with a young child or toddler. Usually made of plastic, they can be broken fairly easily with a powerful budge, which means they are no substitute for supervision, however, this is good to know in the event of a fire or other emergency.
