A man can have two types of erections, an erection from stimulus, or an erection from arousal. The former can turn into the latter. Meaning the latter is what a man has when he's sexually aroused, or horny.
An erection from arousal results from any sexual attraction or thought of sex. It's the most intense form of an erection and typically takes the longest to go away without further sexual stimulation.
An erection from stimulus can result from an unexpected or undesired contact, and or lack of arousal. For example, a woman you don't quite have feelings for, or don't think of in any sexual context puts her hand down your pants and begins sexual stimulation of your genitals. Once sufficiently erect, with continued stimulation, it really doesn't matter whether or not the guy was aroused by the person or not. However, if interest develops, and he is aroused, it makes it feel significantly better.
Now that you know what causes them, we'll work on how they feel.
The easiest way to describe an erection is tension. The penis is, in a way like a sponge, in that when arousal or stimulation begins, the vein constricts filling the sponge-like penis with blood making it firm. The skin becomes taut, there is tension at the base of the penis both in the groin and along the top of the shaft of the penis.
If the erection formed because of arousal, it is typically more intense. It may be a turgid or very rigid erection. In that case the guy may feel a greater sensation of tension on the skin. Some might describe it as hurting, but it doesn't hurt, it's tension.
Most guys will agree, there is a gratifying pleasure to a clothed erection. The additional tension and restriction, not to mention stimulation from the clothing can increase, if not exacerbate an erection. Though the situation may vary as they can be embarrassing.
Beyond this, an erection doesn't feel like much of anything without additional stimulation.
There are multiple types of stimulation a guy feels all at once, The physical stimulation can be broken down into the nerve endings in the skin, and pressure on the shaft. In addition, there a physical response to stimulus that affects the testicles, as well as abdomen, and shaft as a whole.
For this part of the discussion, we'll relate. The penis head and the clitoris are the same thing. In both sexes they are the epicenter of sexual stimulation. The majority of pleasure at the head comes from crown or rim of the head, and on the underside. As a guy gets older, the sensation that forms from the stimulation changes slightly. At a young age it may be described like a tickle, but as you become familiar with the sensation it is less of a tickle and more intense. In addition, the shaft immediately behind the head has a lot of nerve endings. This is because it's the point where the foreskin attaches to the shaft of the penis, so regardless of whether or not the guy is circumcised, it's a part of the shaft he feels sensation in. The rest of the skin on the shaft really doesn't have much in terms of nerve endings and therefore doesn't get much stimulation from friction.
You'll here guys mention girls being "tight" or not. This is due to a second aspect of stimulation for a guy. The rigidity of the erection, the nerves running along the shaft, and the pressure put on the shaft by tightness or constrictive nature of the vaginal muscles creates a whole other sensation, which one is less able to describe. It's a very satisfying, and a very masculine feeling.
The last form of stimulus is actually the result of the physical response to the stimulus. For instance, the moment the head begins to get stimulated, the testicles tighten. This is a direct physical response and as the stimulus continues the tension it causes builds. Depending on how strong the abdomen muscles are, both the stimulus and the tension created by any downward pull of the shaft (by gravity, position, or otherwise) creates a stimulating tension in both the abdomen and shaft.
So what happens with all this tension? Eventually release.
As stimulus and arousal continue, every sensation is amplified and intensifies. The closer to orgasm, the penis becomes more rigid, and the tension builds. Then release. Much like a female orgasm, an orgasm for a man is a wave of muscle spasms and a release of endorphins. The muscle spasms continue for about 30-60 seconds after ejaculation and can be both pleasurable or annoying.
When guys are young, hopefully not your age, but like in early puberty, any continued stimulus after orgasm is painful. While most experienced guys can continue to be stimulated after orgasm. However, depending on the situation, he may need a bit of downtime before he's responsive again. Sometimes as little as 5 minutes, sometimes 15-20. It depends on the guy and the number of times he's ejaculated. After each ejaculation it typically requires a longer and longer downtime before any stimulus will be properly responded to.
That is the best, most complete way I can describe it. It may be more thorough than you wanted, or it may not be thorough enough. Either way, deal with it.
The only other thing I'll add in context of understanding what it's like to have a penis is we tend to think in terms of how sex feels physically to us, and when we are aroused and stimulated, we are driven to getting off. There is no emotional context regarding sex for someone your age and little for anyone older than you. If a guy tells you he loves you whilst having, or anytime after you have sex, he's telling you he loves having sex and how he gets off because of you.
If you really want someone to love you, someone for a real meaningful relationship with, take your time, get to know him, be friends, and let him learn to build an emotional connection with you. Girls are not guys, no matter how much they try they cannot be emotionally disconnected from sex, even when they're trying to be. It isn't a weakness, it's a strength. One that boys eventually learn as they become men and we don't learn it unless we are forced to.
Lesson over.
