what is difference between Bits, Bytes, Megabytes

1 Kb (Kilobyte) is roughly one thousand bytes It's enough to hold a low quality picture, or a small text file.

1 MB (Megabyte) is roughly one thousand Kilobytes Enough to hold five or so high quality pictures, or a large document, maybe a small PowerPoint presentation, or an MP3 with 30 seconds or so.

1 GB (Gigabyte) is roughly one thousand Megabytes It's the standard unit for measuring the capacity of portable storage devices such as thumb drives and SD cards (range from 2 to 32 GB at the time of writing); also DVDs (4.7GB) and Blu-ray (25GB).
A Gigabyte is plenty capacity to store a medium quality full- length movie, or 5-10 MP3 albums.

1 TB (Terabyte) is roughly one thousand Gigabytes (and 1 Million Kb) When one speaks about TB, it's usually in the sense of computer hard disks, i.e. massive storage capacity.
At time of writing, the biggest hard drives on the market are about 2 TB.
2 TB = 2,000 GB = 2,000,000 MB = 2,000,000,000 Kb = 2,000,000,000,000 bytes
(remember, these numbers are simplified for your convenience: to be accurate, 2 TB would be 2048^4 = 2,199023255552)