ERDAS Imagine vs. ESRI ArcGIS

[Yong Kwang]

I'm a complete noob to GIS and I'm not familiar with GIS products. So
I'm wondering how ERDAS Imagine is related to ESRI ArcGIS and how they
compare against each other.

Under which situation would one pick ERDAS Imagine over ESRI ArcGIS,
and vice-versa? Are there some strong features in each product that
differentiate themselves, such that they are particularly suited for
certain tasks?

Are they complementary or are they competing products doing the same
thing?

Googling around tells me that ESRI and ERDAS seem to cooperate and
complement each other in some way by building their software to inter-
operate more easily like supporting each others' file formats. Yet
their GIS products seem to be competing against each other?