CHAR vs. VARCHAR: CHAR is fixed-length, and it will be padding inputs with spaces to the defined length. Works best for short strings (< 5 characters), e.g. codes, like currency (almost always 3 characters), US status (2 chars) etc. VARCHAR on the other hand works best for longer strings and is only storing as much characters as are inserted/updated. If you define a VARCHAR(200) and only insert Christmas into the field, your field occupies 9 characters (and a litte bit of overhead)NCHAR/NVARCHAR: Unicode versions of the above; always stores 2 bytes per characters, so your field with Christmas in it will store 9 characters and use 18 bytes to do so. Those are needed if you have non-Western-European characters - such as Cyrillic, Arabic, Hebrew, Asian or other alphabets.
VARCHAR(MAX) / NVARCHAR(MAX) are the replacements for TEXT and NTEXT - storing up to 2 GByte (2 billion bytes) of data - that's over 300 times the content of Tolstoi's War and Peace - should suffice for the vast majority of cases :-)So your decision tree could be like this:
ID = bigint Telephone = varchar(12) Email = varchar(100) Description = nvarchar(max) (sql Server 2005 and 2008 only) Name = nvarchar(100) SSN = varchar(11) Price = money ShipDate = datetime (date if using SQL Server 2008) Sex = char(1) (i have also used bit before 0 = female 1 =male) Discontinued (true false field) = bit Quantity = int if not fractional decimal if it is fractional ZipCode = varchar(10)