I won't spend long on this, as it's a very simple topic. I'll just give you examples with corresponding HTML code:
Normal text Normal text
Bold text <B>Bold text</B>
Italic text <I>Italic text</I>
Teletype text <TT>Teletype text</TT>
Different sizes of text <FONT SIZE="5">Different sizes </FONT><FONT SIZE="2">of text </FONT>
Different colours of text Different <FONT COLOR="#6300ff"> colours </FONT> of <FONT COLOR="red">text</FONT>
<BLINK>Flashing text</BLINK>