Sorry for not much documentation. I have to work on that...

But basically you create a Cheetah instance like so:

cheetah = Cheetah::Cheetah.new({
  :host             => 'ebm.cheetahmail.com',
  :username         => 'foo_api_user',
  :password         => '12345',
  :aid              => '67890',                  # the 'affiliate id'
  :whitelist_filter => //,                       # if set, emails will only be sent to addresses which match this pattern
  :enable_tracking  => true,                     # determines whether cheetahmail will track the sending of emails for analytics purposes
  :messenger        => Cheetah::ResqueMessenger
})

,and then there are three methods you need to know about:

cheetah.send_email(
  eid,    # cheetahmail's EID for the event triggered email
  email,
  params, # a hash of parameters used to populate any dynamic fields in the email template
)

cheetah.mailing_list_update(
  email,
  params # a hash of parameters to populate the mailing list fields with
)

cheetah.mailing_list_email_change(
  oldemail,
  newemail
)