a muscle car is a  bare bones, high performing vehicle originally geared towards t the youth market 60s and70s.it's usually a mid-sized car with two doors and a real wheel powered tray. It also came with a very powerful and large V8 engine that was sold at a low price. 

The original muscle cars were usually made for  street racing. Some car makers had expensive limited cars and showoff models. Chrysler had one of the first muscle cars  that pumped out 300 hp.

 The C-300 was able to go from 0 up to 60 miles in under 10 seconds and hit speeds over 125 mph which was fast back then.

Muscle cars got popular in the 60s when companies like Plymouth, Dodge  and fords go into street racing. The Dodge dart, another popular muscle car was above to do the 1/4 mile in thirteen seconds! wow super fast.

Muscle cars made from other car makers like Pontiac in cheeky came around in '64
Mopar, specifically came out with its fast Mopar 426 hemi engines and Ford introduced the Thunderbolts too

We sometimes see these muscle cars today but usually  just at the car shows