The now Facebook-owned instant messaging app is the most popular one in the world. As a one differentiating point from Facebook Messenger, for example, WhatsApp does store messages locally, requires you to sign up with you mobile number, supports all the emoji, sticker, and GIF goodies, and is coming with a dark theme very very soon - currently it is under beta testing.
App Description from Google Play
WhatsApp from Facebook
WhatsApp Messenger is a FREE messaging app available for Android and other smartphones. WhatsApp uses your phone's Internet connection (4G/3G/2G/EDGE or Wi-Fi, as available) to let you message and call friends and family. Switch from SMS to WhatsApp to send and receive messages, calls, photos, videos, documents, and Voice Messages.
WHY USE WHATSAPP:
• NO FEES: WhatsApp uses your phone's Internet connection (4G/3G/2G/EDGE or Wi-Fi, as available) to let you message and call friends and family, so you don't have to pay for every message or call.* There are no subscription fees to use WhatsApp.
• MULTIMEDIA: Send and receive photos, videos, documents, and Voice Messages.
• FREE CALLS: Call your friends and family for free with WhatsApp Calling, even if they're in another country.* WhatsApp calls use your phone's Internet connection rather than your cellular plan's voice minutes. (Note: Data charges may apply. Contact your provider for details. Also, you can't access 911 and other emergency service numbers through WhatsApp).
• GROUP CHAT: Enjoy group chats with your contacts so you can easily stay in touch with your friends or family.
• WHATSAPP WEB: You can also send and receive WhatsApp messages right from your computer's browser.
• NO INTERNATIONAL CHARGES: There's no extra charge to send WhatsApp messages internationally. Chat with your friends around the world and avoid international SMS charges.*
• SAY NO TO USERNAMES AND PINS: Why bother having to remember yet another username or PIN? WhatsApp works with your phone number, just like SMS, and integrates seamlessly with your phone's existing address book.
• ALWAYS LOGGED IN: With WhatsApp, you're always logged in so you don't miss messages. No more confusion about whether you're logged in or logged out.
• QUICKLY CONNECT WITH YOUR CONTACTS: Your address book is used to quickly and easily connect you with your contacts who have WhatsApp so there's no need to add hard-to-remember usernames.
• OFFLINE MESSAGES: Even if you miss your notifications or turn off your phone, WhatsApp will save your recent messages until the next time you use the app.
• AND MUCH MORE: Share your location, exchange contacts, set custom wallpapers and notification sounds, broadcast messages to multiple contacts at once, and more!
*Data charges may apply. Contact your provider for details.
IMPORTANT: WhatsApp beta program is currently full and it is not possible to join it until WhatsApp dev will increase the limit. Please contact WhatsApp devs directly regarding the beta limit or download the app from APKMirror (Updated 23.01.2020)
DARK MODE: How to get Dark Mode on WhatsApp beta
If you have additional questions, feel free to ask in our Telegram group 📲.