Optics and photonics are often used interchangeably, although their meaning can vary depending on the context. In fact, an ongoing debate among experts has yet to find a conclusion on what the exact differences are [1]. Here is our take on it: "optics" often refers to the classical concept of light rays, the study of vision, and the transmission, reflection, and absorption of light in different media [2]. In contrast, "photonics" emphasizes the quantum mechanic dual character of light, in particular the "quanta" of light (the photons!) [3]; moreover, it also often refers to the technologies and the industry that involve optics and optical instruments [2].
This, of course, is only a brief summary of online sources, and one could discuss in length more details. If you want to share your thoughts, send us an email!
References:
[1] Optics.org: Optics or photonics: what’s in a name? http://optics.org/article/32348 accessed: 2011-08-30.
[2] New Oxford American Dictionary, accessed: 2011-08-30.
[3] Wikipedia: Photonics http://en.wikipedia.org/wiki/Photonics accessed: 2011-08-30.