That article from Aaron is very good for understanding, also I’d recommend that you read manning publications Spring in Action book, they give very good examples on how the spring solves that problem it will definitely improve your understanding of this.
EDIT :
I came accross this in this great book called Growing object oriented software guided by tests :
Coupling :
Elements are coupled if a change in
one forces a change in the other. For
example, if two classes inherit from a
common parent, then a change in one
class might require a change in the
other. Think of a combo audio system:
It’s tightly coupled because if we
want to change from analog to digital
radio, we must rebuild the whole
system. If we assemble a system from
separates, it would have low coupling
and we could just swap out the
receiver. “Loosely” coupled features
(i.e., those with low coupling) are
easier to maintain.
Cohesion:
An element’s cohesion is a measure
of whether its responsibilities form a
meaningful unit. For example, a class
that parses both dates and URLs is not
coherent, because they’re unrelated
concepts. Think of a machine that
washes both clothes and dishes—it’s
unlikely to do both well.2 At the
other extreme, a class that parses
only the punctuation in a URL is
unlikely to be coherent, because it
doesn’t represent a whole concept. To
get anything done, the programmer will
have to find other parsers for
protocol, host, resource, and so on.
Features with “high” coherence are
easier to maintain.