How to find machine module name for `drush dl`?
According to my experience, I know machine names of some commonly used modules,
but basically when it is for new module or when I have doubt in machine name, 
I google it the module name, it should provide you with link for drupal project page,

the url will be in the format   https://drupal.org/project/[project-machine-name]

drush dl [project-machine-name]


for multiple module you can do like this 

drush dl [project-machine-name1] [project-machine-name2][project-machine-name2] ..