CoinMarketCap is a website that tracks most of the alternative coins that has hit the market as well as Bitcoins and shows users the current value in dollars and Bitcoins for each coin. It also shows the amount of coins that is currently in circulation, the volume being traded the last 24 hours, the change in value over the last 24 hours in percentage and the market cap which is the total value of the specific coins that are in circulation.
The information on CoinMarketCap is updated regularly as they receive feeds from different cryptocurrency exchange sites as well as directly from the crypto coins entrepreneurs, so that the numbers are as close as possible to the correct figures at all times. You can see the movement of the price of a coin in the market graph, showing you the last seven days movement so you can easily see if a coin is getting more or less valuable. The coins are placed in order of the total market cap and not by ability to grow in value, each coins value and so on, so looking at the top coins is not always representative of which currencies is currently growing in value the most.
CoinMarketCap is a good site to follow the movements of all the major and smaller cryptocurrencies, check how many coins are being mined on a daily basis and see the price movement. The simplified overview of everything makes it easy to quickly navigate to the coins you have an interest in and is a great tool to have next to your exchange account.
CoinMarketCap is not in any way a site where you can exchange coins or gather advice regarding coins, so it is simply a tool that you should use together with other places of information, so you can follow the market trends easily and make your predictions when you want to invest in coins and make a profit.
CoinMarketCap has a forum thread in Bitcointalk.org where everybody who has created their own coin are welcome to suggest having their coin added to the website. The owner of CoinMarketCap will then look into the coin, how many that are in circulation, value and all other information he needs and make the decision if he wishes to add them or not. A general rule of thumb is that new coins will be added on a weekly basis, where coins that are considered dead are also removed from the site. In some cases there will be more regular updates, but in general you can expect to see new cryptocurrencies being added on a weekly basis.