Microsoft Excel's native file formats are denoted either by a .xls or .xlsx file extension (.xlsb for new binary based format); .xlt and .xltx extensions are used for templates; visual basic macro-enabled documents and templates are defined by .xlsm extensions starting from Excel 2007.
.xls and .xlt files are OLE structured containers in the Binary File Format. The format is used by Excel 97-2003 and there is still great number of such files available and most Office suits are capable of reading and creating them.
.xlsx, .xltx, .xlsm are based on Office Open XML (SpreadsheetML) format and basically are set of zip-archived xml files. .xlsb is very similar format with cell data saved in binary form (BIFF format). These formats have been introduced with Excel 2007 and used in Excel 2007-2013. Password protected Office Open XML file is OLE container with the actual source file encrypted and attached.
VBA projects can be saved as part of most of the formats, although VBA projects can be considered as "independent" attachment.
Below you can find Thegrideon Software tools designed for Excel documents and templates as well as VBA Projects with basic information about protection strength and recovery methods available.

MS Excel 97 - 2003
File Types:	*.xls, *.xlt, *.xla, ...
Protection:	There are two level of protection available in MS Excel 97 - 2003:  password protection with password to open,  workbook, sheets, shared workbook and write protection.
Strength:	Password to open can be removed with guarantee.  Additional passwords (sheets, workbook, etc.) are recoverable instantly.
Tool:	ExcelDecryptor

MS Excel 2007 / 2010 / 2013
File Types:	*.xlsx, *.xlsb, *.xltx, ...
Protection:	There are two level of protection available in MS Excel 2007 - 2013:  password protection with password to open,  workbook, sheets, shared workbook and write protection.
Strength:	Password to open is based on strong AES encryption, thus exhaustive (brute-force) password search is required.  Additional passwords can be recovered or removed instantly.
Tool:	Excel Password Recovery

VBA Projects
File Types:	*.xls, *.xlt, *.xlsm, *.xltm, ...
Protection:	VBA Projects (macros) password protection - password to open VB code.
Strength:	VBA Project password protection can be bypassed instantly.
Tool:	VBA Password Bypasser