Coinmarketcap's new API pricing
( Ranked by Marketcap Totals )
The Top 10 Altcoin list is all derived from the total net worth of the overall altcoin amount at the time of this page loading.
Since Bitcoin isn't an altcoin, it has it's own spot being the grandfather of all cryptocurrency.
The Altcoin market cap is the total number of altcoins in circulation at the current time, when you add the total number of altcoins in circulation and multiply by the current price of that altcoin, you have the total market cap value. Now mind you, some altcoin's totals are going to differ, due to the various issues like premines, reservations or pre-orders for early funders and even basic errors or typos, so market cap value is not an exact amount but is a good estimatation.
All pricing is obtained via CoinMarketCap.com
Bitcoin & Altcoin's price fluxuates contantly, like the stock market the price is always changing due to supply & demand, as well as rumor, news & speculation. Bitcoin's price is more tied to the world's events, rumors and even further adoption by big name companies. A lot of times, it's country regulations being in favor or against cryptocurrency that tends to move the BTC price the most, it's also speculated that Bitcoin's price will rise before bigger cyber attacks take place, because many times not, hackers & ransomware will ask for Bitcoin in return to decrypt your data, often just to ask for more and more before you might even be lucky enough to get a decrypt key.
Altcoins pricing is a bit more uninvolved on such a global level and more involves the people developing & promoting the altcoin, what it's features are and how they differ from others and what's the buzz going around about it. Many times not have there been cases of an altcoin having the hype of legends only to dwindle into obsurity. It's all a matter of how the team is set up, very much like a company if you want to succeed, you need to be prepared to scale at a moments notice, not as a future concept but as a possible current action, because not having that ability is what will stretch an altcoin team to it's breaking point.
Often it's rumors about an altcoin that becomes the downfall or the boost they were looking for, it's all a matter of how good your digital marketing is. But all too often do you have the issues where scams are the main reason an altcoin is created and that's where things get bad, there is often signs to watch for though, if it sounds too good, if there is a large premine, take a look at the cryptocurrency scam prevention page for more signs to avoid altcoin scams.