One kind of similarity is functional, e.g butterflies have wings for flying and so do bats. [analogy] Another kind of similarity  is structural, but the structures are used for different functions. [homology]

Pennisi, Elizabeth. 1977. Haeckel’s embryos: fraud rediscovered. Science 277 (5 September):1435. Referring to Richardson, Michael K., Hanken, J., Gooneratne, M. J., Pieau, C., Raynaud, A., Selwood, L., and Wright, G. M. 1997. There is no highly conserved embryonic stage in the vertebrates: implications for current theories of evolution and development. Anatomy & Embryology 196: 91-106.