If you develop in quirks mode, then you are effectively asking the browser to emulate pre-IE6 bugs. Is that really something you want to be doing?
Wikipedia explains the basics of triggering quirks mode, but there are a number of special cases it doesn't cover. The link provided by txwikinger gives a good explanation of the history and the main differences.
Jukka Korpela explains the differences in more detail and also how to check the compatibility mode:
Checking the mode To check which mode
(Quirks vs. Standards) a browser is
On Firefox, use the command View/Page Info (and see the General
pane); or if you have Web Developer
Extension, just check an icon in its
toolbar (third icon from the right).
in the address bar, and check whether
the popup window then says CSS1Compat
(indicating Standards Mode) or
BackCompat (indicating Quirks Mode);
alternatively, download and install
the simple Quirks or Standards Mode
It is worth noting that many browsers have two modes other than quirks mode - almost standards mode and standards mode. Unfortunately, IE6 and IE7 only run in almost standards mode and so if you need to support these browsers, it is tempting to try to use almost standards mode in all browsers. However, it is probably better to develop against the standards and then try to fix up browser quirks. HTML5 will only offer the strict mode, so you really should try to develop against this for future compatibility.