One thing that I can't quite get my head around is Reprojection.
It comes up a lot in questions on Stack Exchange, and there are plenty of How to questions and answers about specific issues using various software.
I have searched the web with "What is reprojection?" and I have had a good read. The ESRI FAQ about Projection Basics is good http://support.esri.com/en/technical-article/000005562 - From this...
The terms 'geographic coordinate system' and 'datum' are used interchangeably, but as noted above, a GCS includes a datum, spheroid, units of measure and a prime meridian. The coordinates for data change depending on the datum and spheroid on which those coordinates are based, even if they are using the same map projection and parameters.
...I feel this starts to explain.
I understand Coordinate Reference Systems. I understand the reality that the earth has an irregular shape and the that different reference systems 'tie' theoretical co-ordinates to real places on the planet surface, and that different CRS are needed around the globe.
This GIS SE post is useful for that Understanding concept of Spatial Reference System?
And this answer with a link to a working manual of projections is a great resource Learning about Reprojection?
Again this Wikipedia page starts to touch on reprojection https://en.wikipedia.org/wiki/Wikipedia:Graphics_Lab/Resources/QGIS/Reprojection
...to reproject (warp) an existing lat/long project to a specific projection
What is actually happening when you Reproject the data from one projection, to another, projection. Can it be explained in simple language?