There were 2 versions in earlier times:
First was Netscape Navigator and second was Microsoft Internet Explorer.
Browser in beginning could not start using W3C when it was introduced as it would break most of the websites.
Therefor browsers provided 2 modes to face new standards compliant sites differently from old legacy sites.
Nowdays 3 modes are used by Layour engines in browsers:
1 Quirks mode: In quirks mode, format copies nonstandard conduct in Navigator 4 and IE 5. These were required for sites composed before presentation of web guidelines.
2 Full standard mode: In this mode, the conduct depicted is same as portrayed by HTML and CSS details. A large portion of the cutting edge browsers uses full standard mode.
3 Almost standard Mode: In practically standard mode there is modest number of quirks execution.
Ensure you put the DOCTYPE directly toward the start of your HTML document. Anything before the DOCTYPE, similar to a remark or a XML affirmation will trigger quirks mode in Internet Explorer 9 and more established. The DOCTYPE as, <!DOCTYPE html>, is the least difficult conceivable, and the one suggested by HTML5. Prior adaptations of the HTML standard prescribed different variations, yet all current programs today will utilize full principles mode for this DOCTYPE, even the dated Internet Explorer 6.
How do browsers determine which mode to use?
For HTML documents, programs utilize a DOCTYPE in the start of the document to choose whether to deal with it in quirks mode or principles mode. To guarantee that your page uses full models mode, ensure that your page has a DOCTYPE like in this model:
How to find which mode is used?
Select View Page Info from the context menu, and look for Render Mode.
Press F12, and look for Document Mode.
Main differences between the modes?
Check here for the list of quirks and almost standards mode for essential differences.