First, what is a VPN? VPN allows your computer (or mobile device) to connect to internet through a network located in US (or any other country your VPN service provides) thus your connection is perceived by the client network as from a different country.
You may need a VPN service if you have a censored content in your country. In this case a VPN, you receive the data from the VPN service and you can access any information you want.
Or you may want to access any service that is limited to certain geography. For example certain TV channels and streaming media (Netflix, HULU and etc..) is limited to United States only. If you would like to access this content outside U.S. you may use a VPN service with U.S.A. exit and you will be connecting to source through a VPN network (via U.S.A. exit) and the you will be able to access the content as if you were in U.S.A.
Ace VPN is one of the best (probably the best) available service provider for VPN. You have one of the best prices compared to other prices starting from 4.58$/month for their premium service which lets you stream 50GB/month which is more than enough for an average user. If you are a heavy user and want to use VPN for P2P services you may buy a Ultimate plan for 13.75$/month. You can use Ace VPN from your Mac, Windows PC or from your iOs, Android based smart phone or tablet.