Grammatically speaking, is 'ing' considered a suffix, or is is something else?

Hi! Thanks for the question.

It seems that the "ing" is considered a verb suffix.

Long Island University / Southampton College
http://www.southampton.liunet.edu/

Readinga-Z
http://www.readinga-z.com/newfiles/levels/lesson_plans/l/noise_in_the_night/skillbuilding.html

Dummies.com
http://www.dummies.com/WileyCDA/DummiesArticle/id-1186.html

Search terms used:  
ing suffix

I hope these links would help you in your research. Before rating this
answer, please ask for a clarification if you have a question or if
you would need further information.
                             
Thanks for visiting us.                              
                              
Regards,                              
Easterangel-ga                              
Google Answers Researcher

'-ing' is a suffix that can form the present tense of a verb, (e.g.
ran, run, running); the present participle of a verb (often used as
adjectives e.g. running shoes); or a gerund (Running makes me sore and
tired).