Sigmund Freud believed that the human psyche is made of two basic
drives--
self-preservation and the
libido. The libido was the
psychic energy that drives individuals to experience
sensual pleasure. This is not limited to
sexual pleasure. He believed that humans went through five stages in childhood. If you made it through these stages smoothly you would become a well developed adult. If you dwell to much or too little in any one stage, you might become fixated.
The Oral Stage is first, lasting from birth to about 1 year. Babies get pleasure from sucking. If you become fixated at this stage you might take up thumb sucking, fingernail biting, and pencil chewing (and maybe drug usage, overeating, verbal fluency).
The Anal Stage is next, from 1-3 years. Young children enjoy holding in and releasing urine and feces. Your toilet training is a major issue here--if your parents push you to fast or too slow you might become anal retentive. A fixation here will show up either as extreme orderliness and cleanliness, or as extreme messiness and disorder.
The Phallic Stage lasts from 3-6 years of age. The id impulses focus on the genitals, and the child finds pleasure from genital stimulation. This is about the time you have to worry about Oedipal conflict for boys and Electra conflict for girls. If these are not resolved, you might identify with the 'wrong' sex, and have anger towards the parent of your own sex.
The Latency Stage lasts from 6-11 years. Not much happens. The child develops the superego.
Finaly, the Genital Stage takes place during adolescence. The (ex-)child get pleasure from sex (or at least wants to). With any luck, you pull through this one and settle down into a healthy relationship (marriage, and children of your own).
References:
Infants, children, and adolescents by Laura E. Berk (1993)
And
Psychology by Philip Zimbardo and Ann Weber (1997)
And
My own memories. Any faults in the writeup are mostlikely my own.