I will get to these today. Just imagine the sound you hear is a line stretching left to right, with bass at the left and treble to the right. Now the line is tilted toward the right so the bass is higher (stronger) and the treble lower (weaker). That’s an example of getting darker. It’s not a perfect analogy, since any complex combination of sounds or balance is possible, but in general when something sounds darker you’ll have less influence of the treble.
Generally, headphones are able to reproduce richer low-end audio, since they have larger drivers than earphones and they don't rely on an in-ear seal to deliver sound. Some listeners also find headphones more comfortable to wear, and easier to put on and take off, than they do earphones. This naturally means that headphones are bulkier, and less suited to the gym.
In-ear headphones are like earbuds but are an upgrade to them. These headphones fit into the wearer's ear canal and stay in place with the use of foam or rubber tips. These tips are available in custom sizes so they can fit each individual wearer. When a proper fit's achieved, in-ear headphones reduce outside noise and deliver sound quality at about the same level as over-ear and on-ear headphones.