骑车，指的是摩托车，自行车或者电动车，也可以骑马。可以理解为跨在某个工具上 坐，很容易理解，sit down 坐下，也可以坐火车，坐飞机，坐汽车； 乘坐，乘坐公交车，乘坐有轨电车，乘坐轮船，乘坐飞机，和坐火车是一样的意思

a.骑：骑自行车（ride bicycle), 骑马 （ride horse）骑摩托（ride motor) b.in order to understand “坐” maybe to figure out its relationship with "座” will help you 坐：v. sit or take a seat on a vehicle 座：n. seat c.乘坐： formal degree 乘坐>乘>坐 "乘" sounds like more elegant. People don't use "乘坐” in daliy communitcation. （Sub way, air plane, bus)Operating Company will use it in broadcast to show its respect and profession to customers. “乘” and "坐“ are similar. but according to its syllable，i would like to use"坐 zuo" because it is shorter than "乘 cheng". :)

骑 means to straddle on something(like an animal or a bike) and ride it yourself：骑马、骑车（here 车 denotes two wheels, such as bicycle or motorbike）. So "骑汽车" is incorrect because a car has four wheels and you can't straddle on it. 坐 means to sit on a vehicle as a passenger：坐车、坐公交车、坐火车、坐飞机 乘坐 is a more formal way to say 坐.