Brief summary: the VERB + ing form can cause a lot of confusion when analysed. First of all we are familiar with this form as part of the verb structure commonly know as continuous or progressive like in

We are talking about the continuous form in class at the moment.

Here, structurally, the VERB + ing is combined with a form of the auxiliary verb BE. Whereas the auxiliary BE takes on all the grammatical ‘work’,  the VERB + ing never changes. In traditional grammar terminology it is called a participle, the ing participle or ‘present’ participle. Confusion sometimes arises, I believe, from the various semantical functions this participle – the form VERB + ing – can take.