Where home security hardware is concerned, some of it constitutes real security hardware and some of it is simply designed to provide some privacy. Knowing the difference can help you improve your home security habits.
On your front door, if you've set things up correctly, you'll find a deadbolt, a kick plate and a locking door knob. In this situation, the deadbolt and kick plate are both security hardware. Together, they make it very difficult for anyone to open the door with a kick or a shoulder. The key-in-knob lock, however, is simply a privacy-enhancing device. If someone were to approach the house and simply try to open the door, the key-in-knob lock would let them know it's locked. If that person happened to be a thief and there was no deadbolt or kick plate in place, the key-in-knob lock would do essentially nothing to stop them from opening the door. This should serve to answer any questions about whether or not deadbolts are necessary.
Windows are privacy enhancing devices that can be transformed into legitimate security devices, given the right modifications. For instance, standard glass is not a security device. It will keep someone from entering your home by shattering the glass provided they're no stronger than a toddler. Adding laminate or tempered types of glass to a window, however, make it a much more formidable means of controlling access. A window that is reflective from the outside is a privacy enhancing device, obviously, as well as a security device as it keeps the contents of your home safe from prying eyes.
Screen doors are privacy devices. They allow a homeowner to, essentially, enjoy having their door wide open in the warmer months while still presenting a hindrance to anyone who would simply walk in uninvited. Remember that security value can be assessed, partially, on the difficulty of the obstacle it presents. A screen door that can be simply kicked in is no real obstacle. An iron door with screens added to it to allow it to function as a screen door, however, is a very real security device. Security is about making crime as difficult as possible.
When assessing home security, it's important to note where privacy devices have been used in place of legitimate security hardware. A shed or garage that has only a key in knob lock and flimsy glass windows is probably safe from anyone whose intentions are simply to be rude. If someone wants to steal your belongings, however, those devices will do little to discourage, or stop, them.
An alarm system is probably the device most accessible to homeowners that offers the greatest possible level of security. In any scenario, having a professionally installed alarm system makes it very difficult to break into a home undetected. They also work by taking simple privacy devices, such as windows, and outfitting them with monitors which has the effect of turning a privacy device into a security device in its own right.