The keywords of the Devil include...money matters, feeling the burden of the material side of life and the desire for physical and material things. Feeling of frustration and oppression. A tendency to collect and hoard money and material objects... lust and sexual obsession. Wow, that's a heavy list!
The lesson the Devil tries to teach us is one of simple awareness and human empowerment. Once we begin to realize that darkness or the potential to do evil is a natural part of each and every member of the human race, we can then begin to gain mastery over it. All human beings have the capacity to get the upper hand over this by tapping into the goodness that also dwells within us all.
Once a person becomes properly self aware to the point where they are no longer controlled by the pull of addictions, wants and greed, it allows them to gain freedom from fear of being controlled by other people, as well as other forces that are always at work in the world at large.
When the Devil card makes an appearance in a tarot spread, it is typically letting the querent know that he/she is currently in a situation that has robbed him/her of control over their own life in one way or another... most likely through their own mistakes (or perhaps mistaken perceptions). However, the appearance of this card also signals the possible start of a chain reaction that can lead to no good if it's not nipped in the bud sooner rather than later.
Poorly thought out actions lead to loss of control and loss of control in turn leads to insecurity, depression, and further inactivity. Once elements like that set in and begin to affect a life, it can be very difficult to turn things around.
The solution to this problem is for the querent to reclaim control over his personal power and identity. The Devil can only conquer you if you believe it can. This is a call to leave ignorance, indolence, and materialism behind. True happiness and success results from getting in touch with the spiritual being at one's core and realizing that strength comes from within... not without.