I'm a little confused on this topic. I need to put 2 SAR images into same coordinate grid.

In ESA SNAP tool, there is 2 options:
 
**coregistration:** Radar->Coregistration->Coregistration and 

**collocation:** Raster->Geometric Operations->Collocation

SAR images are from COSMO-SKYMED and ALOS1 missions.

So. What is the conceptual difference between collocation and coregistration?