Expand = to become bigger in size or importance.
Examples:
"His business expanded greatly throughout the previous year.""Balloons expand when you blow into them.""Our little class has expanded into a big school." To expand also means to make something else bigger in size or importance.
Examples:
"He wants to expand his vocabulary." "The government is going to expand the airport." "They will expand the short movie into a mini-series."The word "expend" has a completely different meaning… 
Expend = to use a lot of time, money, energy, etc.
Examples:
"James has expended all his efforts to help his family get out of debt.""Beth has expended huge amounts of money to build her dream house.""We have expended a lot of time and money on promoting our products."In summary, to expand is to grow, and to expend is to use.
