Speedfan reports nine temps, and I don't know how to interpret them.

The temps (and my guesses) are:

Tempx -- Idle/Load -- my guess at what SpeedFan is measuring
-----------------------------------------------------------------------------------------------------
Temp1 -- 34C/32C -- system board (perhaps cooler when CPU fan ramps up?)
Temp2 -- 20C/40C -- a CPU temp (but not a core temp)
Temp3 -- -55C/-55C -- no sensor
Temp1 -- 28C/28C -- perhaps no sensor (no apparent change)
Temp2 -- 30C/30C -- perhaps no sensor (no apparent change)
Core 0 -- 29C/51C -- CPU core 0
Core 1 -- 29C/48C -- CPU core 1
Core 2 -- 30C/53C -- CPU core 2
Core 3 -- 29C/49C -- CPU core 3

I'd appreciate learning the meaning of each temp, and whether they are safe.

P.S. Speedfan (4.47) shows a red indicator when a temp is 50C or higher, but I don't know if a core temp of 50C is too high for an i7-3770S.

The core temps are from the internal core temperature diodes and usually the best ones to keep track of, and for what I've read so far seem to be quite accurate in todays i-core CPU's. Speedfan has the red marking level user configurable and the program has no database over what temps are okay for invidiual CPU's, so you can set that red marking level to whatever you want. I haven't seen official numbers of safe temps for these CPU's but anything below 70C is fine. Their internal over heating protection probably does not kick in until aroun 100C, like previous CPU's. I would keep it under 80C at all times personally. If overclocking, keep track of highest core reading as one might end up running quite a bit hotter than the others, for various reasons.