How can I open an app using Terminal?
You can use open’s -a option:

open -a Mail


You can specify the application path instead:

open /Applications/Mail.app


If you want to use AppleScript (osascript from command line), open app isn't quite equivalent. Instead, you can either use

osascript -e 'tell application "Mail" to activate'


or

osascript -e 'tell application "Mail" to launch'


You can see this question for the difference between the two.