Yes, they are similar. But ‘appreciate’ is the verb and ‘thankful’ is the adjective. You can say, ‘I appreciate your gift’, ‘I am thankful for your gift’, ‘I thank you for your gift’, but you cannot say, ‘I thankful your gift’, ‘Appreciate’ in the adjective form is ‘appreciative’, so you can say, ‘I am appreciative of your gift’. I hope this is helpful. Good luck! Jason

As Jason has explained, 'appreciate' is a verb, while 'thankful' is an adjective. If you want to thank someone for something they have done or have given you - or in other words 'express your appreciation' - 'grateful' is a better word to use than 'thankful'. Contrary to what many people believe, the adjective 'thankful' does NOT have the same meaning as 'grateful'. 'Thankful' means pleased and relieved - for example that something wasn't as bad or as long as you had feared,or that something bad didn't happen. If you want to thank someone, you should tell say, for example: I really appreciate what you have done for me. or I'm really grateful for what you have done for me. Grateful - not thankful.