:CH=CH2, also expressed as H-C=C:
___________________________|__|
___________________________H H
(Sorry about the underscores; the spacing doesn't work if I don't include them)
This compound is a derivative of ethene, which is CH2=CH2. In place of the fourth H atom, CH3 has a lone pair (as shown in the above formula). Or, it can be bonded to another compound as well in that empty spot.