In women's fragrance there are four common categories of scent which are based on the concentration of perfume oil they contain. The higher the oil concentration, the stronger the scent and the longer it will remain. From greatest concentration to lowest, they are:
Perfume (or Parfum): 25% or higher
Eau de Parfum: 15-18%
Eau de Toilette: 10%
Eau de Cologne: 5%
Another factor to bear in mind is that perfumes are comprised of three fragrance notes, a metaphorical reference to musical notes that stand individually and combine harmoniously to form the whole. In reference to fragrance, these notes are referred to as top, middle, and base notes and describe the progressive olfactory impression the scent makes. The notes, in a nutshell:
Top: initial scent that dissipates within minutes
Middle (or Heart): scent that becomes apparent within 15 minute of application and may last an hour
Base (or Deep): scent that becomes apparent 30-60 minutes after application and comprises the longest lasting impression, up to a day.
Now that you know some of the basics, we come to the question of choosing the scent that's right for you. Perfumes are marketed to project a certain image - Chanel N5 is classy, Gucci Guilty is sexy, etc., etc. This knowledge may make you feel different, but it won't change the objective reality of the scent, so try to see beyond the marketing. Visit the perfume section of a beauty or department store. Based on what you know about the intensity and longevity of each category of fragrance, ask directions to what best fits your needs. Use the tester to spray the paper strips the counter provides, and take a whiff to get a general impression. Don't sniff more than three or four, because your sense of smell will be overwhelmed and fatigued. Choose one you like, and spray it on your wrist and behind your ears. To get the best sense (pun) of the fragrance, wait a few hours so you can experience each of the notes. Trust your instincts. And those of your significant other or an honest friend.
Once you've chosen a scent, don't forget to be mindful of the venue. For everyday [work] wear, apply a little lighter than when going out. If your signature scent is very intense, consider applying it to your calves (as weird as that may sound). The greater distance to neighbors' noses might be what civility requires!
Bonus Track: Check out the fragrance wheel below to see how the perfume industry classifies the various scents found in perfumes. You don't really need to know this info to choose a signature scent, but it never hurts to have a vocabulary to communicate likes and dislikes!