The p-bond in an alkene does not permit rotation, thus all of the atoms attached directly to the alkene lie in a plane. Groups attached to the alkene could be positioned on the same side of the alkene or on opposite sides of the alkene. Such compounds are different in chemical and physical properties as well as in their geometry, and are called geometrical isomers. In 2-butene the methyl groups can be located on the same side or on the opposite side of the double bond, giving rise to two geometrical isomers. The isomer with the methyl groups on the same side is called the cis isomer, while the isomer with the groups located on opposite sides is called the trans isomer. Trans isomers of compounds are usually more stable than cis isomers
Or I can simply say that :
Two carbon atoms joined by a single bond are capable of free rotation  about themselves. When two carbon atoms are joined by a double bond,  they cannot rotate freely and the relative positions of the various  groups attached to these carbon atoms get fixed. This gives rise to  geometric isomers. Such compounds, which possess the same structural  formula but differ in the spatial arrangement of the groups about a  double bond are called geometrical isomers. The phenomenon is known as  the geometrical isomerism or (also known) as cis - trans isomerism.

In cis-form, the like groups lie on the same side.


In  trans-form, the like groups lie on the opposite sides.


cis-2-butene              trans-2-butene(boiling point 3.7°C) (boiling point +1°C)
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regards
Ramesh