Yandere is a Japanese term for a person who is initially very loving and gentle before their devotion becomes destructive in nature, often through violence. The term is derived from the words yan (ヤン) meaning a mental or emotional illness and dere dere (デレデレ) meaning to show affection. Yandere characters are mentally unstable, often using extreme violence as an outlet for their emotions.
Tsundere (ツンデレ) is a Japanese character development process which describes a person who is initially cold and even hostile towards another person before gradually showing their warm side over time. The word is derived from the terms tsun tsun (ツンツン), meaning to turn away in disgust, and dere dere (デレデレ) meaning to show affection.
Hopefully, that clears up any confusion (Doug & Nick)