씨 and 씨앗 have the same meaning, a seed or seeds. 씨 is the short form of 씨앗, and is more informal and in wider use. 종자 means the line of blood or genes passed down to the next generation, or the plants or animals carrying them ("stock"). 종 is the hanja character for "species". For example, Darwin's The Origin of Species is translated as 종의 기원. 자 can be thought of as "carrier".