Basically they act all cold and cool on the outside but then have moments of weakness around people they like.
Wiki page if my explanation fails XD:
http://en.wikipedia.org/wiki/Tsundere
Biggest tsundere is Domyouji from Hana Yori Dango XD That's who I think of when tsundere is mentioned. And also tons of anime characters Rie Kugimiya voices for XD