Search for Packages on the Command Line Ubuntu 16.04 with apt-cache and apt

Basic Package Searching with apt-cache

apt-cache search mbstring
php-mbstring - MBSTRING module for PHP [default]
php-patchwork-utf8 - UTF-8 strings handling for PHP
php7.0-mbstring - MBSTRING module for PHP

More Verbose Results with apt

Alternatively, the apt utility provides a bit more information, including identifying packages which are already installed:

apt search mbstring
Sorting... Done
Full Text Search... Done
libapache2-mod-php5/now 5.5.9+dfsg-1ubuntu4.21 amd64 [residual-config]
  (none)

php-mbstring/xenial,xenial,now 1:7.0+35ubuntu6 all [installed]
  MBSTRING module for PHP [default]
php-patchwork-utf8/xenial,xenial 1.3.0-1build1 all
  UTF-8 strings handling for PHP
php5-cli/now 5.5.9+dfsg-1ubuntu4.21 amd64 [residual-config]
  (none)
php7.0-mbstring/xenial-updates,xenial-security,now 7.0.22-0ubuntu0.16.04.1 amd64 [installed,automatic]
  MBSTRING module for PHP