Olympus and Panasonic were the early entrants to a new class of interchangeable-lens cameras much more compact than traditional SLRs. They're both members of the Micro Four Thirds partnership that governs image sensors and lenses, meaning that Micro Four Thirds lenses can be used on either company's camera bodies.
The new membership, like that of third-party lensmaker Sigma before, means photographers can expect more variety for Micro Four Thirds cameras. That's important because right now, traditional SLRs have a much wider range available for photographers eager for fisheyes, supertelephotos, and anything in between.
Competing with the Micro Four Thirds group are several other incompatible lens-mount systems from Nikon, Sony, Samsung, Pentax, and now Fujifilm as well.
Olympus announced Tamron's arrival in the group today. Also joining Micro Four Thirds is another Japanese company, Astrodesign, which sells professional video and testing equipment
Via 4/3 Rumors