Coinbase provides market information and descriptions for 50 cryptocurrencies, determined by current market cap. A small subset of those assets are available to trade on Coinbase. You can find a list of cryptocurrencies available to trade on Coinbase here.
Cryptocurrencies not available for trade on Coinbase are viewable by Coinbase customers for the purpose of education. Coinbase customers can view a description, historic trading data, market cap, and links to relevant white papers and project websites.
NOTE: Cryptocurrencies not available for trading on Coinbase exist for educational purposes only and are selected based on market cap. The inclusion of educational cryptocurrency pages does not indicate that Coinbase will support any specific cryptocurrencies for trading in the future.