Great x Grand: Qual a diferença

The words are similar, but "grand" has Latin roots. It has the idea of "imposing".

"Great" has Germanic roots and is used to refer to non-material things. Material things are often "large" or "big". World War I was called at the time "the Great War". "Great" can mean "impressive" or "important" or "monumental"; however, it can also mean "wonderful."

In Ireland, it's common to hear people use "grand" to mean "wonderful" as well, so in that country, the words would be interchangeable for both meanings.

In Informal English, the word grand also means "mil dólares ou libras".

Now, regarding on your question, the word grand refers to:

1. Great in size, and fine or imposing in appearance or impression; illustrious, dignified, or noble (said of persons); majestic, splendid, magnificent, or sublime (said of things):

A grand monarch.
A grand view.
A grand conception.

2. Having higher rank or more dignity, size, or importance than other persons or things of the same name:

A grand lodge.
A grand vizier.
A grand piano.

You should read: http://wikidiff.com/great/grand

Reference: Pearson Education. Longman Dictionary. 2004.

Just for curiosity, the prefixes grand- and great- are used to denote family relationships that are two or more generations apart.
Either prefix may be used for the aunts and uncles of one's parents and the children of one's nephews and nieces, great- being more frequent than grand-: great-niece; grand-nephew; great-uncle; grandaunt.

>> The prefix grand- is always used for the parents of one's parents and the children of one's children: granddaughter; grandfather; grandchild; grandma;

>> The prefix great- is also used for the parents of one's grandparents and the children of one's grandchildren: great-grandmother; great-grandson; great-grandparent; (the father of one's great-grandfather is one's great-great-grandfather, and so on.)

REFERENCES:
(1) Pearson Education. Longman Dictionary. 2004.
(2) MANSER, Martin H. Good World Guide: The fast way to correct English - spelling, punctuation, grammar and usage. 6th ed. Plain Language Comission. Bloomsbury Publisher. 2007.

Muito obrigado, Bernardi!

Interessante as conceituações que apresentaste.

Eu gostaria de saber, especificamente, por exemplo, em título honorífico: "Great Secretary" e "Grand Secretary". Sabendo-se que o "Grand" é superior ao do título de "Great", estaria correto traduzir por: "Grão Secretário" e "Grande Secretário", respectivamente, no idioma Português brasileiro?

Grato por seu auxílio.

A expressão "grão-secretário", assim como "grão-vizir" e "grão-duque" implica numa posição ou título de nível elevado. A tradução correspondente em inglês seria grand secretary.

Contudo a expressão "grande secretário" é passível de ambiguidade e nisso devemos ter um pouco de cuidado. Na língua portuguesa, eu sugeriria o uso de secretário-chefe (Chief Secretary).

A palavra inglesa -great é um adjetivo e somente expressa uma grandeza em casos de parentesco. Esse status só é visível quando great antecede a palavra grand. Também podemos analisar de outra forma: quando essas duas palavras estão juntas sempre nessa ordem GREAT GRAND-, antecedendo um substantivo.

Compreenda a estrutura base do adjetivo -great aqui.

REFERENCES:
(1) Grand Secretariat.
(2) Pearson Education. Longman Dictionary. 2nd. 2009.