enkidu's writeup is somewhat exaggerated
(and the same, incidentally, applies to much of the
contents of the Japanese honorifics node, which yet
again seems to have been written mostly by people who derive their
knowledge of things Japanese from anime). Back in
days of yore the meaning was indeed lord, but
nowadays, sama is simply a polite "Mr." or "Mrs.",
used when
addressed from someone below to someone above according
to the complex politeness levels of Japanese.
For example, a store clerk always addresses all
customers as sama, another person's family
members are referred to as okusama (your honorable wife) and
okosama (your honorable children), and if I wrote a letter to anybody
I would tack a sama onto the end of their name
on the address.
While sama is generically polite, most people in special positions of power have special titles
and they expect them to be used. A teacher or mentor
of any kind is sensei, while President Bush is
would be referred to as Busshu-daitooryoo,
"Bush-president". In very formal situations the
honorific would be kakka, which is roughly
equivalent to Your Excellency. The reigning
Emperor is referred to by his title alone, usually
Tennoo Heika (literally something along the
lines of "a star from heaven upon his throne on the
earth").
Finally, there are a large number of set expressions
that tack the word sama onto the end:
o-tsukare-sama ("You must be tired"),
go-kuroo-sama ("You've worked hard"),
o-kinodoku-sama ("I feel your pain"), etc.
The sama is a fixed part of the phrase and
I would use these as is even to people "below" me.
(Confused yet? So are most students of
Japanese. Needless to say, this is all just a scratch at the tip of a very large iceberg...)