As has been expected, Samsung will soon release the S5 mini, a smaller and lower cost phone which fits below the S5 in their phone lineup. As with the S3 mini and S4 mini, the S5 mini will be a decidedly mid-range device, not an S5 with a smaller screen.
Specifications include a 4.5 inch Super AMOLED 720p screen, 1.5 GBytes of RAM, 16GB of storage, and a 1.4GHz Exynos 3 processor. The rear facing camera is only 8mp, with the front-facing camera sporting 2.1mp. This compares to the regular S5's 5.1 inch, 1080p display, 2 GB of RAM, 16 or 32GB storage option, a 16 mp camera, and a 2.5GHz cpu. It also appears to lack the fingerprint scanner and heart rate monitor of the "regular" S5. The mini's battery appears to be smaller, too.
It does have the same IP67 rating for water resistance as it's big brother, though, which is a plus compared to it's competition in this segment. IP67 means it's dust-proof, and will withstand submersion in up to 1 meter of water for 30 minutes.
Other than water resistance, this doesn't seem to offer much over the 3 year old Galaxy S3. This writer is still hoping for a flagship caliber phone, with specs along the lines of the normal S5, but in a size that makes one-handed use easy. It doesn't appear that anyone other than Apple is interested in that market, though.