Hey..I keep seeing "Tsundere" and " Yandere"...what do they mean exactly??

here a explication
Tsundere is a word combining two words, "Tsun Tsun" and "Dere Dere". Both terms describe attitudes of a person. "Tsun Tsun" is used for cold/blunt/curt attitude, while "Dere Dere" is used when a person becomes spoony in front of his/her lover.
like the girl in toradora or zero no tsukaima they are mean at first and then nicer

tsundere is someone who is cold to you upfront but actually caring (perfect example is taiga from toradora), and yandere is someone who is sweet and innocent upfront but was actually a psycho (example is yuno from mirai nikki)

(chiefly Japanese fiction) A fictional character who fits the archetype of being genuinely romantic, loving, kind, merciful, sparing, sweet and gentle, but is at the same time brutal, psychotic or deranged in behavior. The psychotic tendency can be both sudden and ever-present. Often used for both comedic and dramatic displays of character
like the girl in mirai nikki

Prickly on the outside, mushy on the inside.

Thanks for the responses everyone :) I appreciate it ^^

Tsundere example
(stop touching me you disgusting creep, even though your thinking please touch me more....)
Yandere example
(because I love you, I'll crave out your eyes you don't see anyone but me) something like that do you understand!!!

tsundere means a person kind of cold with a strong character but shy on some things. yandere is someone who acts cute but can be a psycho... Hope that help thought it is kind of short and not so precise