"Consent" is only legitimate if it is "informed consent." This means that the individual is aware of exactly what they are getting into. This usually comes up in medical settings, where before a person agrees to a procedure they must know the benefits and dangers. Another element of informed consent is that the person must be mentally capable of consenting. For example, giving a severely intoxicated individual a list of benefits and dangers doesn't really "inform" him of anything.
Implied consent is when you don't affirmatively express consent verbally or in writing, but your behavior indicates your consent. For example, if somebody challenges you to a fight, and you don't say anything but you raise your fists, that might be considered implied consent. If somebody asks you to have sex with them, and you rip of your clothes and jump on them, that might be considered implied consent. Implied consent also applies in situations where you aren't physically or mentally capable of affirmatively consenting to something, but people can assume that if you could express consent, you would. For example, if EMS scoops you off the street unconscious, they will treat you without your express consent, because they assume if you could consent to life-saving treatment you would.