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what is raster data and vector data in GIS? what is meant by their conversion to each other?

I need open-source GIS data for the Gamo Abaya Region in Ethiopia.? I am looking for land cover data (ie. geology, vegetation, soil types) that can be inputted into ArcMap. Also, I need remote sensing imagery or high-resolution aerial photography photographs of the region. Hopefully, the rasters are free, but otherwise is okay too! Thank you!

What is the best Windows based open source GIS system? Is it compatible with ARC GIS? Are there sample data sets? (raster & vector)