In a general sense, the word libido is used to describe the basic desire for sex. The term was coined by psychiatrist Sigmund Freud. Originally, he used the word specifically to describe the sex drive, but eventually, he started using it to describe the overall human desire to create. He often compared it to the desire to destroy. This more wide-ranging definition is less commonly used, and when most people talk about libido, they are usually specifically talking about sex drive.
The sex drive has the evolutionary purpose of forcing a species to procreate. Nearly every complex animal has libido in some form or another. With humans, the sex drive is a year-round thing, while most other mammals have a seasonal sex drive. The human sex drive is also very mental and deeply tied to our cultural behavior. Many scientists think that the basic drive for sex is totally biological, but the specifics of what attracts people can be culturally influenced, especially at an early age.
Libido in both men and women is directly tied to testosterone. Men have about 40 times as much testosterone as women, and this is why men are generally thought to have a more intense sex drive and more aggressive behavior. This disparity in testosterone generally exists among other mammals as well, and most species show a bias towards more intense male aggression and sex drive compared to females. Lowered levels of testosterone can be the cause of a lowered sex drive in both men and women, and in some cases, testosterone therapy has been effective in dealing with lowered libido, especially for menopausal women.
As people age, their sex drive often lessens. Many people have looked for different therapeutic solutions to this problem. Certain enhancement drugs were developed for men, but there aren't as many medications available for women. Some people are hopeful that testosterone therapy may be a solution, but it is still being tested.
Age is the most common cause of lowered sex drive, but there are many other reasons. Sometimes, a lowered sex drive may be the result of an illness or drug abuse. Alcoholism is an especially common cause of lowered sex drive. For women, it is generally common for loss of sex drive to come from emotional problems. Females tend to be a bit more psychologically driven in terms of sex, while males are often more driven by visual impulses.