Holding a cryptocurrency position on an exchange is not safe.
In January 2018, Coincheck, a Japanese cryptocurrency exchange, was hacked and $530 million worth of cryptocurrencies were stolen. This is just the most recent notable hack in a series of exchange security breaches:
Mt. Gox was hacked for $460 million in March 2014
Bitfinex was hacked for $72 million in August 2016
Bittgrail was hacked for $170 million in February 2018
Storage Options
The default storage option after purchasing a cryptocurrency is to leave it hosted on an exchange. This is convenient because the exchange does the work of managing your public and private keys and gives you an email and password login (hopefully backed by two factor authentication), which is familiar and convenient. This convenience is nice, but it also means that the safety of your coins relies upon the security of the exchange. While exchanges are continuing to improve security, there will always be serious security risks in leaving coins hosted on an exchange.
Here are better alternatives to leaving coins on an exchange:
>>Hardware wallets: This type of storage refers to a physical device that is offline and manages private keys, a form of cold storage. Hot storage refers to methods connected to the internet (like hosting on an exchange or other website) while cold storage is not connected to the internet. Hardware wallets are a relatively recent development but so far there have not been any verifiable incidents of cryptocurrencies being stolen from hardware wallets and they are regarded as the safest option for storing cryptocurrencies. Two popular hardware wallets are the Ledger Nano S and TREZOR.
>>Paper Wallets: Like hardware wallets, paper wallets are a form of cold storage and host coins offline. A paper wallet is a print out of public and private keys and can be implemented via QR codes. It is recommended that paper wallets are backed up by at least one other copy and stored in a safe location, like a safe deposit box (keeping the private key safe is crucial). In this case, physical access is total access and anyone who gains access to your paper wallet can take your holdings. MyEtherWallet is a popular paper wallet for Ethereum and ERC20 tokens.
>>Desktop Wallets: As the name suggests, desktop wallets are software clients that are downloaded and run from the desktop. While desktop wallets are safer than holding coins on an exchange, being indirectly connected to the internet creates the opportunity for security to be compromised (this is a form of hot storage, which is less secure than cold storage).
>>Mobile Wallets: Like desktop wallets, the name makes the arrangement clear — mobile wallets are applications that are hosted on a mobile device. Just as a desktop wallet can be breached, a mobile wallet may also be hacked (this is a form of hot storage, which is less secure than cold storage).
Conclusion
Be careful managing the storage of your coins and do not hold significant positions on an exchange. Never give anyone your private key. If possible, implement cold storage via a hardware wallet or paper wallet. As always, be mindful of phishers and scammers online and verify any/all URLs before transacting.
I think that Coinomi is a good way to keep your money safe. It is a multi-currency wallet on Android, very useful and convenient. If you didn't have a hardware wallet, it is the best solution for you.