They rise and fall mostly with Bitcoin, Ethereum has shown recently to be more independent, but the biggest drops are usually spurred by panic about crypto regulation.
Regarding 2018, I'm your fool...here are my predictions:
Bitcoin will remain top market cap and crypto numeraire. Majority of the newest projects like IOTA, Tron, etc. will be down significantly.
Burned by vaporware projects, investors will become more cautious about "new" ICOs and enormous token issuances. None will rise as quickly as TRON, EOS, and others did.
Ethereum will remain #2 but at least once will have had a great run against Bitcoin, hitting 80% of bitcoins market cap.
Overall crypto market cap will rise 200%-300%.
Ethereum will continue to have most transactions, enterprise, and developer support. Could change in 2019 and beyond, but has a huge lead right now.
Ripple will crash further, but the platform will gain real world usage.
BAT will rise and support becoming common on enthusiast sites, like tipjar services.
Most of the smart contract chains in development will make no real progress, but I expect at least one "ethereum killer" to become a reasonable alternative.
Disclosure: I own Ethereum and Bitcoin, would buy some others but maxed out on my crypto exposure.
I want to play too! Here is my prediction for 2018:
Just as 2015 was the year of the altcoins, and 2017 was the year of the tokens (why mess with a software project when anyone can create an ERC-type token effortlessly, plus there's no mining so you can sell every last bit of it day one), 2018 will be the year of the forkcoins. Things like Ethereum Classic and Bitcoin Cash are just too lucrative to pass by.
Expect to see Ethereum Black, Bitcoin Plus and Litecoin Gold. Each with a subtle change from the parent in block times, hard caps or other parameters. More than one will also try to steal old unspent UTXOs (including Satoshi's) for the developers themselves.
Every improvement to the underlying protocol will see a lot of manufactured dissent to lay the groundwork for a fork, even regarding opt-in features (such as segwit and p2sh).
Ethereum and Monero continue to grow in market cap as they work and nail a niche very well.
Bitcoin core starts to decline in about 3 months as volumes on Bitcoin Cash grow. Lightning doesn't pan out because it has strong centralizing forces so it is no better than fiat, and it will still have higher fees than Bitcoin Cash because it adds use, rather than limits it, which further increases demand, and therefore fees.
Bitcoin Cash grows as it takes the mantle from Bitcoin Core.
The market hasn't noticed that real innovation and development is taking place on this chain yet. It has adoption coming in from business that depended on the cash use-case and built for bitcoin core. Bitcoin cash also has features coming that were planned for a low-fee environment, like colored coins.
The success of Bitcoin Cash over bitcoin is an inevitable consequence as the public will prefer a store of value they can transact to one that can't be moved. If you make 20 transactions with $400 in Bitcoin Core, because of fees, it is gone! There will be more forks, but none will gather critical mass.
One or two of the newer crypto speculations delivers and succeeds. The others do not. Most of the DAG cryptos are dependent on "goodwill among men", vs. relying on greed, so if they take the spotlight, they will fall to attacks.
Regarding 2018, I'm your fool...here are my predictions:
Bitcoin will remain top market cap and crypto numeraire. Majority of the newest projects like IOTA, Tron, etc. will be down significantly.
Burned by vaporware projects, investors will become more cautious about "new" ICOs and enormous token issuances. None will rise as quickly as TRON, EOS, and others did.
Ethereum will remain #2 but at least once will have had a great run against Bitcoin, hitting 80% of bitcoins market cap.
Overall crypto market cap will rise 200%-300%.
Ethereum will continue to have most transactions, enterprise, and developer support. Could change in 2019 and beyond, but has a huge lead right now.
Ripple will crash further, but the platform will gain real world usage.
BAT will rise and support becoming common on enthusiast sites, like tipjar services.
Most of the smart contract chains in development will make no real progress, but I expect at least one "ethereum killer" to become a reasonable alternative.
Disclosure: I own Ethereum and Bitcoin, would buy some others but maxed out on my crypto exposure.