Words

Appreciation
Main Entry: ap·pre·ci·a·tion
Pronunciation: \ə-ˌprē-shē-ˈā-shən, -ˌpri- also -ˌprē-sē-\
Function: noun
Date: 1604
1 a: judgment, evaluation; especially : a favorable critical estimate b: sensitive awareness; especially : recognition of aesthetic values c: an expression of admiration, approval, or gratitude
2: increase in value

Thanks
Main Entry: thanks
Pronunciation: \ˈthaŋ(k)s\
Function: noun plural
Etymology:
Plural of Middle English thank, from Old English thanc thought, gratitude; akin to Old High German dank gratitude, Latin tongēre to know
Date: before 12th century
1 : kindly or grateful thoughts : gratitude
2 : an expression of gratitude —often used in an utterance containing no verb and serving as a courteous and somewhat informal expression of gratitude

Sorry
Main Entry: sor·ry
Pronunciation: \ˈsär-ē, ˈsȯr-\
Function: adjective
Inflected Form(s): sor·ri·er; sor·ri·est
Etymology:
Middle English sory, from Old English sārig, from sār sore
Date: before 12th century
1 : feeling sorrow, regret, or penitence
2 : mournful, sad
3 : inspiring sorrow, pity, scorn, or ridicule : pitiful

Grateful
Main Entry: grate·ful
Pronunciation: \ˈgrāt-fəl\
Function: adjective
Etymology:
obsolete grate pleasing, thankful, from Latin gratus — more at grace
Date: 1552
1 a: appreciative of benefits received b: expressing gratitude
2 a: affording pleasure or contentment : pleasing b: pleasing by reason of comfort supplied or discomfort alleviated

Extract from http://www.merriam-webster.com