"Amber" refers to two things: a set of molecular mechanical force fields for the simulation of biomolecules (which are in the public domain, and are used in a variety of simulation programs); and a package of molecular simulation programs which includes source code and demos.
Amber is distributed in two parts: AmberTools15 and Amber14. You can use AmberTools15 without Amber14, but not vice versa. See below for information on how to obtain Amber14.
The Amber14 package builds on AmberTools15 by adding the pmemd program, which resembles the sander (molecular dynamics) code in AmberTools, but provides (much) better performance on multiple CPUs, and dramatic speed improvements on GPUs. In this release, more features from sander have been added to pmemd for both CPU and GPU platforms, including performance improvements, and support for extra points, multi-dimension replica exchange, a Monte Carlo barostat, ScaledMD, Jarzynski sampling, explicit solvent constant pH, GBSA, and hydrogen mass repartitioning. Support is also included for the latest Kepler, Titan and GTX7xx GPUs.
AmberTools consists of several independently developed packages that work well by themselves, and with Amber itself. The suite can also be used to carry out complete molecular dynamics simulations, with either explicit water or generalized Born solvent models. The sander program is now a part of AmberTools.