According to the GNUpg website
GnuPG comes in two flavours: 1.4.11 is the well known and portable standalone version, whereas 2.0.18 is the enhanced and somewhat harder to build version.
The release notes say
1.4.11 is the stable version of GnuPG. (2.0.18 is the unstable development version).
GnuPG-2 has a different architecture than GnuPG-1 (e.g. 1.4.5) in that
it splits up functionality into several modules. However, both
versions may be installed alongside without any conflict. In fact,
the gpg version from GnuPG-1 is able to make use of the gpg-agent as
included in GnuPG-2 and allows for seamless passphrase caching. The
advantage of GnuPG-1 is its smaller size and the lack of dependency on
other modules at run and build time. We will keep maintaining GnuPG-1
versions because they are very useful for small systems and for server
based applications requiring only OpenPGP support.
If you read through the notes for each release you will build up a good picture of what features are in 2.x that are not in 1.x. E.g. support for smart-cards.