The words are very similar, and can be synonymous depending on how they are used. To host an event usually means that you are going to provide the resources necessary to have the event, whether food, facilities, or services. The Olympics are a good example, maybe. In 2008, China hosted the summer Olympics in Beijing. It provided the hotels, restaurants, police, stadiums, and the other things that were necessary for the Olympics to be held there. The International Olympic Committee held the event in Beijing. They are the ones who make the rules; they are the supreme authority concerning the Olympic Games. In this example, the host and the one holding the event are different. But many times the ones holding an event are also the ones hosting the event. Beijing could host (and hold) an international trade expo in order to try to attract new businesses to the city, for example. Hope this helps a little.

they are very similar and can probably be used for the same thing, but I think to host means that that person will personally entertain the guests. Whereas to hold means you just provide the place but you don't necessarily entertain them yourself.