An HTML validator checks to make sure the HTML code on
Custom Web Design web page complies with the standards set by the W3 Consortium (the organisation that issues the HTML standards). There are various types of validators - some check only for errors, others also make suggestions about your code, telling you when a certain way of writing things might lead to (say) unexpected results.
A CSS validator checks your Cascading Style Sheets in the same manner; basically, most will check them to make sure that they comply with the CSS standards set by the W3 Consortium. There are a few which will also tell you which CSS features are supported by which browsers (since not all browsers are equal in their CSS implementation).
There are numerous other validators around, both free and commercial, focusing on various aspects of ensuring that your code will run trouble-free across browsers and platforms. You can find a list of free ones (including specialised validators like those that check your code for accessibility) from Free HTML Validators, CSS Validators, Accessibility Validators at
http://www.thefreecountry.com/webmaster/htmlvalidators.shtml 
