Sex work is a consensual transaction between two or more adult persons, whereby sexual acts may be purchased or traded for other services. There are different forms of sex work which involve varying degrees of (physical) contact, for example, erotic dancing, web camming, pornography, phone sex, engaging in penetrative/oral/anal sex etc. Note that not everyone who engages in these types of activities may consider themselves sex workers.
There are many different reasons for engaging in sex work – some people engage in sex work to supplement their income or in exchange for gifts, some may do it for fun or pleasure. Note that trafficking is not the same as sex work, which is a form of sexual exploitation, although it should be noted that trafficking is a complex phenomenon and the lines can often be blurry (e.g. some people may agree to go to a certain country and know that they will be engaging in a form of sex work, while others may be tricked into thinking they are going to be working as a nanny).