NCBI-geo differences between platforms,series and samples
The platform at NCBI-GEO means the particular microarray that was used (because originally most of GEO was just microarrays), OR, more recently, it could mean the type of NGS machine that was used.  For example, every microarray manufacturer creates an ADF (array data file) that maps the DNA sequence at every position on the Chip to an accession ID. And this ADF mapping file gets registered at GEO as a GPLxxxxxx. This was presumably to facilitate direct comparisons between different experiments from various labs using identical Chips (e.g. Affymetrix, or Nimblegen).

With the advent of NGS the Platform has to be a name from an approved list of companies and devices: ILLUMINA, HiSeq, etc.

A series is constructed by the data providers and allows them to cluster the results from related samples. So, for example, in the modENCODE consortium, I believe all of the C. elegans RNA-Seq from Bob Waterston's lab/Project are clustered together under one overarching GSExxxxxxx. More details at www.modencode.org and links therein.