Generally they are the same but the differences are major. A badminton racket is a lot lighter than a tennis racket and the strings are a lot thinner. You would never be able to hit a tennis ball with a badminton racket because it will break. Tennis rackets are also made of much sturdier materials like graphite.
The difference between a badminton racket and tennis racket is that racket for tennis is longer. The webbing area is much larger than the one used for badminton. There is also a longer shaft for the grip area for a tennis racket.