There are three broad categories of consent:
You should note that the standard is "agreed to participate" -- not "wanted to participate" or "enjoyed participating." You can agree to (consent to) things you don't want (doctor visits, annoying jobs, family reunions), and whether or not you enjoyed the activity has no bearing on whether or not you consented to the activity.
By the by, these categories can apply to more than just sex -- there's such a thing as Non-Consensual Bondage, for example, and Non-Consensual Touch, Non-Consensual Body Modification, and so on. And even kissing should be negotiated out ahead of time (nonverbal negotiation is fine). But just to keep things simple, we'll be talking about sex.
Wait, one thing first: You can't consent to something without knowing, to some degree, what it is that you're consenting to.
When you go in to surgery, you have to sign a consent form. Say the surgeon is giving you liposuction, a cosmetic surgery to make you look nicer. Great! That doesn't mean that the surgeon has the freedom to give you other kinds of surgeries (boob job, nose job, dental repair) as long as you're under. She does have the freedom (and the responsibility) to take action if something goes wrong, and obviously you wouldn't know ahead of time what steps she might have to take in order to save your life, but that doesn't give her carte blanche to mess with your body.
In similar vein, just because you said yes to sex doesn't mean that anything goes. There are many activities that might make up a sexual encounter (oral, vaginal, or anal sex; various types of toys; deliberately inflicting pain by slapping/whipping/pinching/biting; involving other participants (threesomes, voyeurism/public sex); etc.). Most of these things need to be negotiated: Is your partner willing to try anal sex? Does he appreciate using pain to heighten sensation? Would she be willing to have sex while someone else is watching? Etc.
If you agreed to participate in the sexual encounter, and there was no significant point at which you withdrew your consent and the other parties kept going, and you basically understood the boundaries of the activity that you had consented to (even if you didn't know the specific particulars), then the encounter was consensual (on your part, anyway).
Note that you can have various reactions to the encounter, not all of them good: You were bored, or annoyed, or angry, or resigned, or even crying while it was happening. But so long as there was some ongoing dialog of consent (doesn't have to be verbal), so that your partner(s) wasn't oblivious to signs that you might want to stop, that's fine. People can have bored sex; they can have angry sex, and they can cry while having sex. We can consent to all of that.
Still, we might aim for a higher standard:
Here's a comic strip from Oh Joy Sex Toy, detailing the highest aims for consent: Everyone's happily agreeing to the activity, and when anyone wants to stop a particular part of the activity, the other party notices immediately and stops right away.
We should all be aiming for that! I just don't think it's that simple (humans are messy, and human relationships doubly so). And verbal consent is good, but the absence of verbal consent is not inherently bad. People are perfectly capable of conveying consent through nonverbal means, and we've been doing so for thousands of years.
At any rate, if all parties involved are happy to be there, fully engaged in the activity, respectful of their partners' boundaries, and focused on the shared enjoyment of all parties involved, that is the very highest standard we can aim for as far as Informed Consent.
Now imagine the circumstances by which you might engage in consensual sex, but with less enthusiasm:
So there are a lot of possible situations in which you are consenting to the encounter, and you know what you're in for, and you're not forced or coerced, but you're also not exactly enthusiastic about it either. Some of these scenarios can even take place within a fully committed, loving relationship (marriage or not), and for very understandable reasons. Key Point: Mood does not determine consent.
On the flip side, we have encounters that you definitely did not consent to. If a non-consensual encounter involves sexual penetration (dick in mouth, dick or objects in vagina or anus), then it's rape (although the legal definition varies, and sometimes doesn't even count "made to penetrate" (a woman raping a guy)). There are other activities that can be Non-Consensual (e.g. non-consensual groping is Sexual Assault).
As a side note, it turns out that if you ask people "have you raped anyone?" they'll almost always say no (even on anonymous questionaires), but if you describe acts that are rape without using the term "rape," people will admit to it: Have you ever had sex with a person who was drunk? Have you ever badgered someone for sex until they gave in? Have you ever had sex with someone who was under your authority (a student, patient, ward, etc.)? Etc.
Fandom, however, has subdivided stories about rape into two categories: Rape and NonCon. And here it gets a little interesting.
There is a distinction between stories where the rape experience is eroticized, and stories where it is not.
There is a distinction between stories where the rapist wanted to hurt the victim (or just didn't care about them), and stories where the aggressor (probably still a rapist) wanted to help the victim, or thought that the victim secretly wanted it.
If the rape is not eroticized, and the rapist wanted to hurt the victim (or just didn't care about them), then this is a Rapefic. Generally, though not always, it's a Rape Recovery fic, where the rape is the Hurt part of a Hurt/Comfort fic, and the victim has no further relationship with the rapist.
If the rape is eroticized, and the rapist wanted to help the victim (or thought that the victim secretly wanted it), then this is a NonCon scenario. Generally, though not always, it leads toward a relationship between the two.
Now, fandom is a bit split about how to classify the other two quadrants: If the rape is not eroticized, but the aggressor cares about or wants to help the victim, is that Rape or NonCon? And if the rape is eroticized, but the rapist wanted to hurt the victim (or just didn't care about them), is that Rape or NonCon?