As Angular6 has been released, They have added a new command ng add .

what will be the difference between existing  command npm install <package> and ng add <package>

ng add

ng add <package> uses your package manager and installs the dependency. That dependency can have an installation script which can be used to do more job except the dependency installation. It can update your configurations (In angular.json file as well), download another dependencies based on that one or create scaffold templates (with initial markup and logic).

For example you run the command ng add @angular/material?—?Install, it will automatically install the package and configure in angular.json file too.

npm install

npm install <package> just installs the dependency. It means, it  will only install your package into your project but will not configure in order to use.

For example you run the command npm install jquery it will install jQuery in your project but you need to configure manually in .angular-cli.json file (as in v5)