What is difference between big tag and bold tag ?

Answer 1

In addition:

<b> is an older tag style oriented, while <strong> is a newest, semantic oriented...
Actually both are commonly displayed as more weight text ( bolder ), but in future the second may be styled differently.

Answer 2

<big>Hello</big> will increase the font size making the text bigger.
similar to how <small>Hello</small> will decrease the font size making it smaller.

<b>Hello</b> or <strong>Hello</strong> will increase the font weight making the text bold.