What is meant by 20 dB signal-to-noise ratio? I'm reading a paper in which a discrete signal

$$x(n) = s(n) + w(n)$$ 

is considered. $s(n)$ is a known, deterministic series, and $w(n)$ is white noise with zero mean. The authors write that


  signals were generated with an SNR of 20 dB 


What does this mean?


What does signal energy mean? There seems to be several ways of defining this, but no attempt to do so is done in this paper.
What does 20 dB SNR mean? I suppose that either $20=10\log{E_s/E_w}$, or $20=20\log{E_s/E_w}$, but am not familiar enough with these concepts to be sure.
A discrete signal is often interpreted as an analog voltage signal. The signal x(n) should be considered as a sampled version of x(t) with a unknown sampling frequency.

Q1: 

So when we're talking about the energy of the signal, it is presumed that the voltage signal is obtained in a circuit with an 1Ω resistor in series, so that v(t)^2/R = P.

Q2:

Signal-to-noise ratio is used in many areas and is defined as the logarithmic power ratio of the signal and noise, i.e. SNR = 10*log(Ps/Pw). The other SNR = 20*log(Ps/Pw) is not true, because the Ps and Pw are power values. The latter is used when the signals are represented as "voltage" signals, i.e. SNR = 20*log(s(n)/w(n)) = 10*log(s(n)^2/w(n)^2).