Oops, Your question was two pronged , OK, I'll explain the verbiage to you:
Semiconductor specs usually include as a minimum, two types of data: Absolute Maximum and operating values. The absolute maximums are the "red lines" of operation, somewhere above this the part will self destruct. In many applications you may not be able to reach some of the maximum limits as you will have exceeded another.
Note that all the specs that look like "Vabc" are the voltage between the two terminals a and b when the other terminal is at condition c.
Vceo = Voltage collector to emitter with base open
Vcbo = Voltage collector to base with emitter open , easy to measure with a zener diode tester, but no real circuit uses a transistor this way. Note that if you short the base to gnd, and the emitter is grounded too, then the maximum voltage between collector and emitter is numerically the Vcbo value. So any real circuit like yours, with some resistance in the base leg will have an effective Vce somewhere between the 40v and 60v value for a 2N4401. The significance of Vceo is that the collector-base junction behaves as a zener diode, and the leakage current will sharply increase when the voltage exceeds a certain value. This leakage current increases with temperature, so what the manufacturer is telling you is that, provided the supply voltage is < 40v then there will never be enough leakage current flowing into the base to accidentally turn on the transistor.
When the supply voltage exceeds 60v, the leakage current across the collector- emitter junction will be sufficient to heat up the junction, even if you short the base to ground.
Ic = collector current , this heats up the transistor, and can cause localised hot spots. Ic is the maximum continuous current, be aware the test conditions are nothing like real life, so always ensure your value is lower than half this.
Pd = power dissipation, this is the power level that will cause the junction to exceed 175C with specified cooling applied to the transistor, your circuit may not have the same conditions, may be crowded, may be in high ambient, so plan on only using half of this. Note that Pd is determined by the package, and not what is inside, so all devices in a TO92 package will have the same Pd, even if if its physically impossible to get this power level.
Ft is the transition frequency, this is the frequency at which the common emitter current gain drops to unity. Above this frequency, a piece of wire has more (common emitter current) gain. So for example you wouldn't use this transistor for circuits operating faster than 25MHz, Your relay operates in 5mS so no problem.
Hfe This is one of the "H" parameters , assumimg the transistor is a black box with a pair of wires going in and another going out. So its "H parameter" "forward" "common emitter" so put 1mA into the base, ground the emitter and 100-300mA appears at the collector. Note that Hfe changes with the operating point, so it's never a fixed number.
Vce(sat) is the voltage across the transistor when its "hard on" , probably at the 600mA mentioned, this gves 600mA x 0.4V = 240mW, so they have arrived at Ic by working backwards from the Pd. Vce(sat) also varies with operating point.
Vz: This is the voltage at which 1mA flows through the zener, this changes slightly with temperature.
Iz: The nominal operating current (this drops as the zener voltage goes up )
Pd: is 1W , all parts with this package have a 1w rating, this assumes certain lead lengths, PCB areas etc, zeners will happily operate close to this value, but will get very,very hot, and may damage surrounding parts. Best to go for half this value.
Note that multiplying 20mA x 12v is only 240mW , but you will find that zeners 10- 20v will be 20mA, and 20v to 40v will be 10mA, manufactures just bunch them up in tables. Zeners always fail short circuit, so only operate them close to the limits if a short circuit does not cause a cascade failure.
Thermal impedance: Not mentioned by you, but is seen in data sheets. Basically it takes a certain time for the junction to heat up, so a short pulse that exceeds the maximum continous rating can be safely applied in certain circumstances. So for example the transistor in your circuit will pull several amps to charge the capacitor briefly, not a problem for a 1nF capacitor, but "briefly" becomes too long for a 1000uF capacitor. Which is why your schematic raised the eyebrows of your readers.