Primary Market
The Corporates, who wish to raise the capital, go to public through primary market. It is a place where companies will contact public.The Companies will issue the share to Public in primary market. This process is called IPO (Initial Public Offering). Public will subscribe for those shares.
Say for example, if a company issues one lakh shares. If there is huge demand for those shares, the subscription numbers will be more than the number of shares issued. This is called oversubscribed shares. If the subscription is less than the issue then it is undersubscribed. Once the company raises the capital through IPO, the shares will be traded in Secondary market
Secondary Market
Secondary market is where the shares are traded. People who already hold the shares will sell it for Higher/Lower price, depends on their need and that companies performance. Buyers will buy the shares from the sellers. This trading will be done based on the demand and supply of the shares.Summary
Issuing of shares happen only once in the primary market. After that, is will be traded in the secondary market among the buyers and sellers. So if you missed buying those shares in the primary market, it will be available in the secondary market. You have to buy it from the one who already bought the share for a higher lower price based on the demand for the share.