Signal is a messaging app by Open Whisperer Systems, just like WhatsApp or iMessage or Facebook Messenger, but one that's geared towards privacy and security. It's encrypted, simple, and free.
Signal Vs WhatsApp
They both use the same underlying encryption "the Signal Protocol" but drastically differ in terms of underlying philosophy.
The only data Signal retains is the phone number you register with and when you last logged into their server. That's it. It doesn't even record the hour, minute, or second - only the day.
Signal is open-source and funded by donors, on the other hand WhatsApp is owned by Facebook, leaves users to being nothing more than another data collection point for Facebook for ad-targeting. WhatsApp was a messaging app first and added the security features later. Signal was all about security first. Because of this, WhatsApp has more now-traditional chat features. Both Signal and WhatsApp also include voice calling and video calling to round out the messaging apps.
Signal has plenty of features too including GIFs, can send your location, and Signal even has disappearing messages.Even voice calls on Signal are encrypted and screenshots can be disabled.
Signal, the non-profit organization, points out that "there are no ads, no affiliate marketers, no creepy tracking", and it's hard to ask for much more than that in terms of privacy. And It's recommended by Edward Snowden. When you have an alternative which doesn't need you to compromise on privacy, why not to switch to Signal.