Here's one illustration of distributed and decentralized systems:

Distributed computing has a long history in computer science ("became its own branch of computer science in the late 1970s and early 1980s").
A distributed system is a model in which components located on
networked computers communicate and coordinate their actions by
passing messages.
With this definition, a decentralized system is also a distributed system.
A key point about decentralization, is that there's no central point of control. Large Internet applications and services are distributed, but most are centralized because the company running them can alter or stop the system.
Ethereum is a distributed platform, as well as a decentralized platform. The platform allows developers to build decentralized applications, as opposed to distributed applications which run on some proprietary cloud.
Above has tried to explain in simple terms. In technical terms, this comment summarizes the challenge of these terms.
These are the most common accepted definitions of terms. You will note
that these describe properties that are related, but neither
orthogonal, nor in a linear spectrum.
decentralized systems: system where components operate on local
information to accomplish goals, rather than the result of a central
ordering influence (this is about decision locality)
federated systems: a cohesive unit formed of smaller sub units which collaborate
to form the whole, but which retain significant local autonomy. (this
is about retaining some autonomy)
distributed systems: system in which
computation is distributed across components, which communicate and
coordinate their actions by passing messages. The components interact
with each other in order to achieve a common goal. (this is about
communication and message passing)
peer-to-peer or peering systems:
systems in which a set of peers are equally privileged, equipotent
participants in collaborative goals. (this is about equipotency)
You
can look these up in {dictionaries, wikipedia, research papers, and
more}. You can see that these refer to similar and overlapping
concepts, but not a spectrum. We can shoehorn these into subsets
in terms of the properties we care to highlight, but there is no singular
spectrum or hierarchy here.