They first appeared in the 1967 story "The Macra Terror" in which their home planet was colonized by humans but the Macra took control of the colony and hypnotized the humans into mining gases for them. They were defeated when the Second Doctor effected an uprising on the planet and destroyed the control center with an explosion, killing the Macra in charge and apparently dooming the Macra on the planet.
They later reappeared over 40 years later in the 2007 episode "Gridlock." Over the coarse of billions of years, The Macra devolved into feral beasts that survived on New Earth, by feeding on the poisonous fumes and stray people who came down in their cars to the "Fast Lane" of the Motorway in New New York. They were apparently again doomed when by 5,000,000,053, the Tenth Doctor opened the roof of the Motorway, allowing the cars to escape to the Upper City, depriving the Macra of food and gases.
But Macras did not perish: following the opening of the Motorway, the Macra got a "new career" cleaning up the city's pollution. The Cybermen also used the Macra to dispose of all non-humans. It is a win-win and all-ends-well for everyone.
The sapient species of Macra were larger than humans, able to hold a human in one claw. They were stocky in build, with large eyestalks, large claws (with a second, smaller set beneath the mouth) and were a dark, mucky brown colour. They were also covered in hairs.
The predatory species of Macra were much larger in size, several hundred times larger than a human. They were more animal-like, having fallen into evolutionary decline.
They had gigantic claws, large enough to easily hold an entire New Earth car. The rest of the body was smaller in proportions, with glowing eyes on thin stalks and six spindly legs. The Macras were predators who hunted by movement and showed no sign of intelligence. They also had distinctive and terrifying roars.