VESA stands for Video Electronics Standard Association. It is a non-profit organisation led by a board of directors that represent over 150 corporate members worldwide. It was started in 1989. It is an organisation that sets standards for PC, workstation, electronics and related innovations like TV brackets.
When it comes to TV brackets mounting, the VESA is often referred to as VESA MIS (mounting interface standard). This standard was first introduced in 1997. It is a way to determine the number of holes and the pattern they appear in behind relevant equipment, such as LCD TVs and the brackets themselves. Some TV brackets meet multiple versions of standards.
The easiest way to find that whether you have the right bracket with you is by checking VESA guidelines. VESA also provides standards for thread type and depth.
Standard dimension for the brackets are from 75 mm x 75mm to 800mm x 400mm. These standards are for the fixing points behind the TV. A 35 ˝ LCD TV wall mount can usually use a 200mm x 100mm plate. If a brand purchased by you does not have compatible VESA numbers; in that case you can use adapter plates to set the mount on it.