The Appearance section in your admin is where the look of your website is controlled.
Themes are templates that dictate and control the look and feel of your website. In building your website we have developed a theme that displays your site exactly how you want it to look. Changing your theme could mean your website breaking and we strongly advise you to not touch these.
Customise is the area where, depending on your theme, you can change things such as your header logo, colour scheme, background image, etc. It is very doubtful that you will ever need to change these, but should you wish to we advise great caution as some changes could break your site. Contact us before you play with this section!
Header. You may or may not have this as a section under Appearance, it really depends on how your theme has been set up. However, if you do have it then we again advise great caution as changing anything here could break your site. Contact us before you play with this section!
Widgets are small blocks of content that you can add to your sidebar, front page or other areas of your website, dependent upon your theme.
Menus is the section where you control what is shown in your menu areas.
Editor allows you to edit the files of your theme. Unless you know and understand PHP, HTML and CSS we recommend you never enter this section!