If you use nodejs you probably used npm before.
Yarn is something similar, It is another package manager like npm.
NPM and Yarn are package managers that help to manage a project’s dependencies. A dependency is, as it sounds, something that a project depends on, a piece of code that is required to make the project work properly. We need them because managing the project’s dependencies is a difficult task and it quickly becomes tedious, and out of hand when the project grows. By managing the dependencies, we mean to include, un-include, and update them.

npm: It is the default package manager for the JavaScript runtime environment Node.js. It consists of a command-line client, also called npm, and an online database of public and paid-for private packages called the npm registry.

yarn: It stands for Yet Another Resource Negotiator and it is a package manager just like npm. It was developed by Facebook and is now open-source. The intention behind developing yarn(at that time) was to fix performance and security concerns with npm.

The differences between npm and yarn are explained below:

Installation procedure

npm: npm is installed with Node automatically.
yarn: To install yarn npm have to be installed.
npm install yarn --global
The lock file

npm: NPM generates a ‘package-lock.json’ file. The package-lock.json file is a little more complex due to a trade-off between determinism and simplicity. Due to this complexity, the package-lock will generate the same node_modules folder for different npm versions. Every dependency will have an exact version number associated with it in the package-lock file.
yarn: Yarn generates a ‘yarn.lock’ file. Yarn lock files help in easy merge. The merges are predictable as well, because of the design of the lock file.
Installing global dependencies

npm: To install a global package, the command template for npm is:
npm install -g package_name@version_number
yarn: To install a global package, the command template for yarn is:
yarn global add package_name@version_number

License Checker

npm: npm doesn’t has a license checker that can give a handy description of all the licenses that a project is bound with, due to installed dependencies.
yarn: Yarn has a neat license checker. To see them, run
yarn licenses list

Fetching packages

npm: npm fetches dependencies from the npm registry during every ‘npm install‘ command.
Yarn: yarn stores dependencies locally, and fetches from the disk during a ‘yarn add‘ command (assuming the dependency(with the specific version) is present locally).
Commands changed in yarn after npm

command: npm & yarn
Install dependencies: yarn
Install package_name npm install package_name
npm install package_name@version_number
yarn add package_name@version_number
Uninstall package:npm uninstall package_nameyarn remove package_name
Install dev package:npm install package_name –save-devyarn add package_name –dev
Update dev package:npm update package_name
npm update package_name@version_numberyarn upgrade package_name
yarn upgrade package_name@version_number
View package:npm view package_nameyarn info package_name
Global install package:npm install -g package_nameyarn global add package_name`

Personal Opinion:
Though my opinion doesn't matter because you may choose which one suits you still I wanted to share what I like.
I like yarn because It is lightweight fast and good for publishers as well.