The definition for a Triplet in music is simply “Three notes in the space of two”. So if you have Eighth note triplets in music, then its “3 Eighth notes in the space of 2 Eighth notes”.
Since 2 eighth notes equal 1 quarter note (1 beat), then Eighth note triplets are three notes played evenly in one beat.
There are tons of examples of them in all the music you listen to everyday – but you never even realize it. An example of triplets in music is Beethoven’s Moonlight Sonata. Listen to the piece and realize the count (1-2-3 4-5-6) or as it can also be counted (Tri-p-let Tri-p-let)
As for quarter notes – it’s “3 Quarter notes in the space of 2 Quarter notes”, etc.