This is a trick question, because there really is no difference - the clitoris extends into the vagina and is the pleasure organ that can be stimulated both from the outside and the inside.

However, the sensations of external clitoris stimulation and vaginal stimulation differ a bit.  Most women can achieve an orgasm by external clitoris stimulation.  Fewer women will achive orgasm from penetration only.  A combination of the two is usually the most effective way to reach climax. Some women experience the vaginally induced orgasm as deeper and fuller. Most women enjoy orgasms from external clitoris stimulation.