In the early days of the web browsers were purely text-driven and could interpret the majority, but not all, of the signs on a typewriter keyboard that we all take for granted. In order that everything should be available HTML codes were written, using the acceptable characters, that would display the others. In today's circumstances this is not so important but there are still browsers in use that need these codes to display the characters.
The most common are:
& which is written as & or &
' which is written as '
" which is written as " or "
£ which is written as £ or £
< which is written as &#60; or &lt;
> which is written as &#62; or &gt;
   (a space!) which is written as &#160; or &nbsp;
© which is written as &#169; or &copy;
® which is written as &#174; or &reg;
° which is written as &#176; or &deg;
º which is written as &#186; or &ordm;
¼ which is written as &#188; or &frac14;
½ which is written as &#189; or &frac12;
¾ which is written as &#190; or &frac34;
⁄ which is written as &#8260;, &divide; or &frasl;
º which is written as &#186; or &ordm;
 
This is not a comprehensive list, there are many others but they are in the main characters from foreign languages.