Are there different antibiotics for different kinds of infections?

I was told that there are different antibiotics that are prescribed depending on what kind of infection you have. I have a really bad skin infection that was caused by several insect bites and have been on several different antibiotics but the outcome was unsuccessful with each one. My physician has recently started me on Cephalexin cap 500mg (Keflex). I'm hoping this one will work and clear up my skin infection. If this is true, that there are different antibiotics for different infections, my question ,of course, would be, is Keflex the right antibiotic for a severe skin infection?