感激 (gǎn jī) emphasizes the internal feeling of being grateful, while 感谢 (gǎn xiè) is the act of expressing thanks (e.g., writing a thank-you note). 感恩 (gǎn ēn) implies a deeper, often spiritual or moral thankfulness (e.g., for life or fate). Use 感激 when you want to focus on the emotion rather than the act of thanking.