Libido is a person's 'sex drive'. A person's overall sexual drive (desire for sexual activity) varies greatly.
Sex drive is influenced by biological, psychological and social factors. Sex hormones like testosterone, and neurotransmitters like dopamine affect the libido. Both of these work on a brain nucleus called the nucleus accumbens (one in each half of the brain). These centres work on motivation, learning, and reinforcement.
Social factors, such as work and family, and internal psychological factors, such as personality and stress, can affect libido. Sex drive can also be affected by medical conditions, medication, lifestyle and relationship issues, and age (e.g. puberty).