It can be a little confusing...
If your new the the area, or just started cast netting for bait, it can be confusing knowing just what your catching. So many names for the same two baits, whitebait, greenback, pilchard, horse minnow, shiners, thread-thins. I guess it just depends on the neighborhood your from. Here is the scoop on these local baits:
The two most common baits are the Scaled Sardine, and the Thread Herring.
Scaled sardine: Silver body with darker back gray, blue, or green. Body is thicker/heaver than the thread thin, making for better casting distance. The eye is twice the size as the thread thins. Scales do not easily fall off, and with a good running live well they will last all day. Easy to keep on the hook, and stays alive for quite awhile.
Thread Thin: At first glance it looks like the scaled sardine, however when compared it's easy to tell the difference. They have the silver body, however it's much thinner. It's belly is deeply curved compared to the scaled sardine. Part of the dorsal fin has what looks like a long piece of thread towards the rear, hence the name. The scales shed easily, and despite a good running live well they don't last long especially in the warmer months. Easy to throw off the hook, and dies fairly quick.