Dictionary.com is a great free online resource: foster verb (used with object) 1. to promote the growth or development of; further; encourage: to foster new ideas. 2. to bring up, raise, or rear, as a foster child. 3. to care for or cherish. nurture   verb (used with object) 1. to feed and protect: to nurture one's offspring. 2. to support and encourage, as during the period of training or development; foster: to nurture promising musicians. 3. to bring up; train; educate. noun 4. rearing, upbringing, training, education, or the like. 5. development: the nurture of young artists. 6. something that nourishes; nourishment; food. http://dictionary.reference.com/

Both words are very similar in their definition. Foster. Encourage the development of something Nurture. Care for and protect (someone or something) while they are growing The main difference I see, is Foster is a word used for a parent or parents taking a child which is not their own into their home and treating them like their own.