Sexual characteristics are the features of the body that distinguish the two genders. The difference between primary and secondary characteristics is largely based around whether the feature is required for reproduction. Non-human animals also have primary and secondary sexual characteristics.
Primary sex characteristics are sexual organs, also known as genitals. Secondary sexual characteristics are other features that distinguish between the sexes and/or signify sexual material but are not necessary for reproduction. Secondary sexual characteristics can also play a role in sexual selection; in other words in attracting mates. For the most part primary sex characteristics are present from birth while secondary sexual characteristics develop during puberty, though this isn't always considered a definitive distinction.
The category of primary sexual characteristics covers all organs in the body that play a role in reproduction. Among males this includes organs used to produce and store sperm (such as testes), organs used to produce ejaculatory fluid (such as the prostate) and organs used for sexual intercourse (such as the penis). Among females this consists mainly of the vagina, the uterus and the ovaries. The classification usually also covers the parts that join these organs together, namely the cervix and fallopian tubes.
The body has many more secondary than primary sexual characteristics. Examples among males include an enlarged larynx, increased muscularity, broad shoulders and abdominal and chest hair. Examples among females include enlarged breasts, broad hips and greater fat around the midsection. Breasts are usually classed as a secondary sexual characteristic even though breastfeeding could be classed as a later stage of the adult female body's participation in the reproductive cycle. Some secondary sexual characteristics are common among both genders, most notably pubic and underarm hair.
Most mammals have primary sexual characteristics that are fundamentally similar to those of humans; key exceptions include some mammals having a bone in the penis and a particular type of mammal called marsupial (such as kangaroos) having two vaginas. Primary sexual characteristics among non-mammals can vary significantly, for example among creatures that reproduce by laying eggs. Many non-human animals have distinctive secondary sexual characteristics such as lion manes or peacock feathers.