from stackoverflow:
"With version 2007 onwards, Access includes an Office-specific version of Jet, initially called the Office Access Connectivity Engine (ACE), but which is now called the Access Database Engine. This engine is fully backward-compatible with previous versions of the Jet engine, so it reads and writes (.mdb) files from earlier Access versions. It introduces a new default file format, (.accdb), that brings several improvements to Access, including complex data types such as multivalue fields, the attachment data type and history tracking in memo fields. It also brings security and encryption improvements and enables integration with Microsoft Windows SharePoint Services 3.0 and Microsoft Office Outlook 2007
In addition, ACE provides a 64-bit driver, so can be used on 64-bit machines, whereas JET cannot.
The driver is not part of the Windows operating system, but is available as a redistributable.[11] Previously the Jet Database Engine was only 32-bit and did not run natively under 64-bit versions of Windows.
As for the second part of your question, I recently installed Office 2010, and I had to download the ACE components separately. I got them from the link Microsoft Access Database Engine 2010 Redistributable. This is likely because I had installed a 32-bit version of Office under 64-bit Windows; in any case, the necessary files are easy to obtain from Microsoft."
from link:
Jet 引擎，可以访问 Office 97-2003，但不能访问 Office 2007。
ACE 引擎是随 Office 2007 一起发布的数据库连接组件，既可以访问 Office 2007，也可以访问 Office 97-2003。
另外：Microsoft.ACE.OLEDB.12.0 可以访问正在打开的 Excel 文件，而 Microsoft.Jet.OLEDB.4.0 是不可以的。
所以，在使用不同版本的office时，要注意使用合适的引擎。
以上对access同样有效。
Jet连接语句：Provider=Microsoft.Jet.OLEDB.4.0;Data Source=|DataDirectory|\您 的数据库名.mdb;Persist Security Info=True;Jet OLEDB:Database Password=您的数据库密码
ACE连接语句：Provider=Microsoft.ACE.OLEDB.12.0;Data Source=|DataDirectory|\您 的数据库名.accdb;Persist Security Info=True;Jet OLEDB:Database Password=您的数据库密码