What is the difference between ArcGIS, GEOMATICA and ERDAS-IMAGINE ? What are the different features of each software?

Hello!

ArcGIS is a powerful software that oriented on GIS, Spatial data modelling, GIS data processing, GIS Data Analysis and Visualization

PCI Geomatica is a geospatial image processing suite that allows for image analysis, imagery manipulating and information extraction from imagery

Erdas Imagine is a powerful remotesensing and photogrammetry software that includes the feaatures from PCI geomatica and more 

So ArcGIS is more oriented on GIS and have some tools for RS (the powerful GIS tool in the world)

PCI Goematica and Erdas Imagine are oriented on RS and photogrammetry and have some GIS tools (but Erdas is most powerful but also most expensive)

Hope that my answer will help you to understand the differents between them.