Helium or He to give it its periodical name is a noble gas that has many uses in modern society and many people might encounter it every day. It is lighter then oxygen or the air we breathe (which is a mixture of chemicals) and is the second most abundant element in the entire universe behind hydrogen. Because of its non flammable nature and the fact that it is so light it is used in such applications as balloons on both large and small scales and is used in deep water diving for pressurization as well.
After the Hindenburg disaster in the 1930s, Helium rather then hydrogen because the gas of choice for all commercial airships and hot air balloons, because of it being a lot less dangerous. Hydrogen and helium have similar properties for the most part as they both weigh less and air, except that hydrogen is extremely flammable and helium is fairly nonreactive in most cases. From here it has also more recently been used for smaller balloons for various celebrations etc, which is what makes helium filled balloons float as opposed to air filled ones which are the same weight as the atmosphere and so sink (as the rubber of the balloon is heavier than the air of course). It is also used in many different manufacturing and chemical processes including the production of magnets, and in rocket technology.
The most noticeable effect on the human body if you were to breathe some in from a pure source would be that your voice becomes much higher, and remains at this timbre for a few seconds until the gas is expelled from the body. Because it contains no oxygen we cannot breathe it properly, and it is possible to cause asphyxiation from breathing it in for too long. It in itself is not actually poisonous, which is why we can breathe it in at all, unlike many other gasses, but it is often impure when intended for industrial purposes, and so may contain traces of other more dangerous gases, which is why it can be dangerous to breathe it in. Other then this though it has no noticeable effects on the human body as such, other then the high voice immediately after having been breathed in.