It's mainly a matter of history, effectively ACE has superceded JET:
Wikipedia answers your question in great detail.
The most relevant sections are:
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.
