Your target customers are those who are most likely to buy from you. Resist the temptation to be too general in the hopes of getting a larger slice of the market. That's like firing 10 bullets in random directions instead of aiming just one dead center of the mark--expensive and dangerous.
Try to describe them with as much detail as you can, based on your knowledge of your product or service. Rope family and friends into visualization exercises ("Describe the typical person who'll hire me to paint the kitchen floor to look like marble...") to get different perspectives-the more, the better.
Here are some questions to get you started:
Are your target customers male or female?
How old are they?
Where do they live? Is geography a limiting factor for any reason?
What do they do for a living?
How much money do they make? This is most significant if you're selling relatively expensive or luxury items. Most people can afford a carob bar. You can't say the same of custom murals.
What other aspects of their lives matter? If you're launching a roof-tiling service, your target customers probably own their homes.