Asset classes describe different types of investment vehicles and are generally categorised according to the risk and return associated with each.

Low Risk
*Cash 
Money placed on deposit is a low risk, low return investment.

* Gilts / Bonds can be of two types:
Government Gilts
These are interest paying bonds issued by governments. Generally they are regarded as the safest form of investment. 
Corporate Bonds
The same as government gilts, except as the name suggests they are issued by companies. As such, they entail a higher risk; in return they tend to pay higher interest rates.

Medium Risk
* Property
Investment can be either direct or via pooled funds

* Alternative / Structured Funds
These are complex and some can be high risk. However, there are others which seek to achieve returns in a diversified manner, and as such have a medium risk.

High Risk
* Shares (sometimes called Equities)
You can invest directly in companies, or via equity funds. 

* Commodities
Covers minerals (gold, copper) to agricultural (wheat, etc). Again, investment can be direct or via funds.

Currency
This is an asset class, but even if an investment is not directly in currency, it can also affect all of the other asset classes. Currency considerations must therefore be taken into account in all investment decisions.