sex: biological characteristics that identify individual organisms as male or female; also the act of sexual intercourse
gender (in relation to people): socially constructed roles, behaviours and activities associated with males and females, individually or in a group
The girls laughed in a very feminine way. [gender]
Whereas the characteristics of sex do not differ substantially between cultures, the characteristics of gender may differ greatly between cultures and between individuals within a culture.
gender (in relation to language): term used in languages (eg French, German, Spanish) in which nouns can take a different definite article to distinguish them as masculine, feminine or neuter. This use of the word is much older than its use in relation to maleness and femaleness in people