what is the hacker?

Any one who programs just for the love of programming.

not really:

From Jargon File (4.3.0, 30 APR 2001) [jargon]:

hacker n. [originally, someone who makes furniture with an axe] 1. A
person who enjoys exploring the details of programmable systems and how
to stretch their capabilities, as opposed to most users, who prefer to
learn only the minimum necessary. 2. One who programs enthusiastically
(even obsessively) or who enjoys programming rather than just theorizing
about programming. 3. A person capable of appreciating {hack value}. 4.
A person who is good at programming quickly. 5. An expert at a
particular program, or one who frequently does work using it or on it;
as in `a Unix hacker'. (Definitions 1 through 5 are correlated, and
people who fit them congregate.) 6. An expert or enthusiast of any kind.
One might be an astronomy hacker, for example. 7. One who enjoys the
intellectual challenge of creatively overcoming or circumventing
limitations. 8. [deprecated] A malicious meddler who tries to discover
sensitive information by poking around. Hence `password hacker',
`network hacker'. The correct term for this sense is {cracker}.

The term `hacker' also tends to connote membership in the global
community defined by the net (see {the network} and {Internet address}).
For discussion of some of the basics of this culture, see the How To
Become A Hacker (http://www.tuxedo.org/~esr/faqs/hacker-howto.html) FAQ.
It also implies that the person described is seen to subscribe to some
version of the hacker ethic (see {hacker ethic}).

It is better to be described as a hacker by others than to describe
oneself that way. Hackers consider themselves something of an elite (a
meritocracy based on ability), though one to which new members are
gladly welcome. There is thus a certain ego satisfaction to be had in
identifying yourself as a hacker (but if you claim to be one and are
not, you'll quickly be labeled {bogus}). See also {geek}, {wannabee}.

This term seems to have been first adopted as a badge in the 1960s by
the hacker culture surrounding TMRC and the MIT AI Lab. We have a report
that it was used in a sense close to this entry's by teenage radio hams
and electronics tinkerers in the mid-1950s.

There are lots of different answers to the term. That is just the one I learned, so I don't have to quote some jargon dictionary.