According to Freud, there are five stages of Psycho-sexual Development. The five stages are: Oral, Anal, Phallic, Latency, and Genital.
As a parent, with a background in psychology, I have been able to watch my children pass through each of these stages.
At birth, infants are oral creatures. The survival depends on eating. The only way to gain nourishment, typically, is through the mouth. Therefore, this stage makes complete sense as existing as stage one in a child's development.
The anal stage begins when the child begins toilet training. While I do not see the extent of Freud's point in this stage, that the child becomes obsessed with the expulsion or retention of feces, one can see that the child's attention is moved from the oral stage to that of the anal one. Many children refuse to train immediately. It is a long process where the child must learn how to recognize the fact that they need to use the toilet.
The third stage is Phallic Stage. This stages emerges as a child begins to become more familiar with their own bodies and questions arise about the bodies of others. Still young, many of these children are able to recognize the difference between genders. Questions arise though because they cannot yet understand why there are differences. Curious by nature, some children may become (in severe cases) obsessive regarding the genital area. I looked at a study where a small child, three years of age, masturbated constantly. While many are put off by this, it is simply the child recognizing pleasure.
The fourth stage, not considered typically psycho-sexual, is the latent stage. It is during this period where sexuality simply goes unnoticed. This typically happens during the pre-puberty years. The child has no concern for anything but being a child. The knowledge of sexual pleasure is unknown--or forgotten from the Phallic stage.
The last stage, and the stage many remain in for the majority of their lives, is the Genital Stage. It is during this stage where puberty has already begun and curiosity begins to erupt in regards to the act of sex.
While I agree with the passing through each of the stages, I do not see eye-to-eye with Freud in regards to the problems that can arise if a child is caught in a stage.