Mahakala is a form of Protector of all Tibetan Buddhism.
His description varies somewhat according to the different teaching lineages. For example, there is the two-armed, big-mouthed Mahakala Bernakchen of the Karma Kagyu, the four-armed Mahakala who is protector of the Drikung Kagyu, and six-armed Mahakala of the Gelugpas described below. They are not all wrathful forms of Chenresi [Skt.: Avalokiteshvara], although
"There is an emanation of Avalokiteshvara which arose in the form of Mahakala and this is the Shangpa Kagyu Mahakala with one face and six arms, in a standing posture. This form was later adopted by Tsongkhapa and followers as the main protector of the Gelugpa School.
Mahakalas can arise from various sources namely Vajradhara and the Anuttarayoga Tantras but are not generically Avalokiteshvara."
~ Jeff Watt, Himalayan Art
It is commonly but incorrectly held that there are 75 variants. There is paradoxically, however, a white one that is associated with prosperity, and they also include a eunuch and also some feminine forms.
Students do not generally do practices associated with dharmapalas such as Mahakala until they have completed the Preliminary or Foundation Practices and of course, after receiving the empowerment to do so.
By the way, the Sanskrit descriptive name, Mahakala (Great Time or Great Dark One) is also used to refer to Lord Shiva, the Hindu god whose tandava dance sustains, but can also destroy, the universe of appearances, and who is associated with Time, the other meaning of kala. [The teaching known as the Kalachakra or Wheel of Time, is a Buddhist tantric system thought of as the key to reality that unites the universe, time and the breath of life.]
Origin of Mahakala
The compassion of the red Buddha Amitabha manifested as Avalokiteshvara who took a vow to forgo his own enlightenment until all the realms of samsara had been emptied.
This vow required a renewal of determination, and so with Amitabha's blessing, Avalokiteshvara next assumed a form with eleven heads and a thousand arms. Still he had been unable to benefit even a few beings.
Therefore after reflecting for one whole week, he determined
that by assuming a wrathful form he would be able " to subdue the
degenerate beings of this Age of Darkness." Also he saw that even
beings who practiced Dharma were unable to escape from the Bardo realms
(time between rebirths where beings may face great anxiety and terrifying
experiences) and he thought that in wrathful form he could also protect
them in that way. And lastly, he thought that the beings in
this Dark Age were poor and needy, experiencing only suffering after
suffering, and that in wrathful form he could provide them an antidote to
that suffering so that by simply making the wish (for protection) their
needs could be met.
These three motives made his
determination even greater than before and so from the heart of Noble Avalokiteshvara
emerged a dark blue HUNG syllable that immediately became
the Instantaneous Protector of Wisdom, Mahakala.
The foundations of all the Pure Lands shook with six kinds of earthquakes, and the Conquering and Transcending One of Immeasurable Light (Amitabha) and all the other Tathagatas of the ten directions proclaimed with one voice:
"Son of the family, it is well that you have made this resolution. You shall have the empowerment of all the wisdom dakinis. You shall have the strength of the wrathful Yama, Lord of Death. You shall have the mountain spirits, the yakshas, the devils and the demonesses as your messengers. You shall embody the great wrathful empowerments of the Body, Speech, Mind, Qualities and Activity of all the Buddhas throughout the three times."
Ever since, bodhisattva Mahakala is the Dharma (Buddha's
Doctrine) Protector of all Buddha fields. 
Read about
the Mahakala of the Sakyapas: how Mahakala went from India to
Tibet.
Four-armed Mahakala (Chaturbhuja) called in Tibetan, Gonpo Chakdrupa or Gonpo ChakZhipa, is related to the Chakrasamvara tantric tradition.
Symbolism of Mahakala in the 6-armed
Manifestation:
The Protector's body is midnight blue, symbolic
of the changeless Dharmakaya.
His three eyes symbolize his knowledge of the past, present and future, and also the manifestation of the three bodies of Buddha.
The crown adorned with five skulls symbolizes the transformation of the five poisons of anger, desire, ignorance, jealousy and pride into the five wisdoms.
His six arms symbolize the attainment of the six Perfections: generosity, patience, morality, diligence, meditation and wisdom. The kartika or triku [or drigu, pron. gigu] the ritual curved knife, cuts attachment to ego.
The kapila or skull bowl filled with blood symbolizes the subjugation of the maras or evil ones. (An alternate interpretation can be found in other contexts.)
The rosary symbolizes his continuous activity for the benefit of beings.
The damaru or hand-drum symbolizes his power over the dakinis. (Also, different interpretations in other contexts.)
His trident symbolizes his power over the three kayas - the spheres of desire, form and formlessness. (An alternate interpretation can also be found.)
The lasso binds those who break their vows.
His two feet are the means and the wisdom to accomplish his task. That his left leg is straight and his right leg bent symbolize his accomplishment of the benefit to oneself and to others. He tramples on a vinayaka, to symbolize his destruction and dispersal of great obstacles.
The sun on which he stands symbolizes his illumination of the darkness of ignorance.
His lotus seat symbolizes purity undefiled by samsara.
The surrounding blazing fire symbolizes his activity that consumes neurotic states.
The tiger skin stands for purification of desire; the elephant skin for purification of pride, and the snake, for the purification of anger.
His other ornaments together symbolize that he has all the
qualities of a Buddha.
This material derives
from a Gelugpa sadhana (ritual) of Tara that includes an offering to
Mahakala. The whole was produced in Jan. 1994 by Dharma Therapy
Trust under Venerable Geshé Damchö Yönten is available unedited at
http://www.lamrimbristol.demon.co.uk/download/tara_puja.htm
Some of the implements used by Dharmapalas such as Mahakala, and
other wrathful manifestations include:
Sword (T. rtse-mdun,
Skt. khadga) symbolizes the wisdom, knowledge or ability to cut through
delusion or obstacles.
Flags, standards and banners
(Skt. dhvaja) which represent the victory of Buddhist teaching over
delusion.
An elephant goad or ankh (T. icags-kyu, Skt.
ankusa) for taming desires.
Spears (T. mdun) that fix or
pin down.
A hammer (Skt. mudgara) mace or club (Skt. gada)
that crush opposition.
The bow (Skt. ripa) and arrows
(Skt. sara): action at a distance.
A vajra staff
(Skt. vajradanda)
Trident (T. rtse-gsum, Skt. trisula)
symbolizing the Three Jewels.
A lasso (Skt. pasa) that
constrains negative forces.
~ list by Chad Sawyer for Mirrors of the Heart-mind at Kaladarshan Arts, Ohio State U.
Different teachers give variant explanations for these weapons or tools.
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Contrast Mahakala Bernachen in a contemporary tangka at Tibet Shop, UK