Earbuds are the modern-day equivalent of stereo headphones. While headphones used to fit around your ears like ear muffs, they now are drastically smaller and fit inside your ear. This provides better sound quality, more change for bass and treble, and blocks out any interfering noise that may be hurting your enjoyment of the music you're listening to. If your earbuds become faulty or stop working, you'll need to replace them. This can be done at any major electronics retailer.
Consider the price. All earbuds are not created equal. A £6.40 pair of earbuds may be affordable, but the sound quality will undoubtedly suffer. But do you really want to spend £65 on a top-of-the-line pair of earbuds? There are some other things you need to consider before you can answer this question.
Consider the type. There are two types of earbuds: the kind that rest on your inner ear, and the kind that physically push into your ear. The latter are called "noise cancelling" earbuds. Because of the way they rest in your ear, they can cut out all outside noise. Some people may find noise cancelling earbuds uncomfortable. Conversely, the kind of earbuds that rest on your inner ear may not provide as good of sound quality.
Consider what you'll be using them for. If you just want a pair of earbuds for working out, you don't have to go crazy and buy super expensive ones. If you'll be using them for long car rides in which you'll be the passenger, however, you may want a pair of noise cancelling earbuds.