Securities are all investments that are intangible or in other words are immaterial and represented by some paper or similar rights. To be easier understand lets mention few financial securities: stocks, bonds, investment funds are the main securities.
Securities are main investments for most of the world's investors because are easiest to reach for investing. Securities maybe listed or not. Listed security means that the security is traded on some stock exchange. There are many stock exchanges on the world and some are smaller some bigger. Large stock exchanges have thousands of stocks on their lists traded and all of them are accessible for investors.
Similar case is with bonds. Bonds are fixed income securities that pay some coupon (interests) for investors. Bonds also can be listed or not and it is not big difference for investor. The only difference is that bonds listed on exchange are easier and faster to buy or to sell that unlisted. However, unlisted bonds or stocks can be also traded.
Unlisted bonds or stocks usually are traded on OTC market. OTC market is not very known by its name because not everyone knows what is an OTC market, but in reality everyone faces the OTC market. The main thing about OTC market is that this market is represented by any participants which are trading in between some securities or goods. An object of OTC market can be anything or any security. Every deal that is made not in centralized market as stock exchange is an perfect example of OTC market.
For example if friend of yours sells his car to another friend without contacting another market participants then he makes a deal in OTC market. The same case is with securities traded on Over-The-Counter trading market. Some participants of the investment market sells their securities to another participant of the market and do not reaches the stock market directly. It means one security just goes from one investor to another, of course the payment goes too just in another direction. This is the key about OTC market and securities. For example most of the investment funds aren't traded on stock exchange and they are distributed on OTC market.
If you are beginner at investing then you should forget about OTC market for securities, because you may be wrong in price of the investment. The thing about the stock exchange that it provides for you expected market value and you can't miss by very far paying the market price. Maybe it will decrease or increase but the fact that you are paying market price is important against that price of the deal in OTC market is only a result of negotiation and the price may be strongly mistaken from the real value.
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