Assonance: Assonance is the repetition of similar vowel sounds, generally close together, used for effect.
It is a form of rhyme which uses a group of words that use the same, or similar vowel sounds for effect. (slow, mellow)
Example 1 From everyday speech:
Assonance can be found in Tongue twisters: e.g.
Example 2 From literature:
Move him into the sun -
Gently its touch awoke him once,
At home, whispering of fields unsown.
Always it woke him, even in France,
Until this morning and this snow.
If anything might rouse him now
The kind old sun will know.
from Futility by Wilfred Owen
NOTE: Look at the way Wilfred Owen uses the repetition of the vowel sounds of "oh" to give this poem a melancholy feel.