Answer:
A half note is a note that lasts two quarter notes. For most songs, it does last half a measure, but this is not always the case. An 8th note is a note that lasts half of a quarter note, and is usually (but again not always) an 8th of a measure. A quarter note is the most common type of note in a measure.
If you want to find out what exactly that stuff means, well, you're out of luck here, but it's probably better to just look that up anyways.