Analog signal can be discrete time? I am new to this site and field too.I always thought analog is continuous and digital signal is discrete. I read this today and got confused about analog signal


  Analog can be 
  continuous time(CT) or discrete time(DT). A discrete time signal is not a digital signal; a 
  discrete time signal is one in which the amplitude is a continuum, and not discretized. On 
  the other hand, if the time is discretized, this signal is still analog. Only when a discrete 
  time signal is passed through an A to D converter, it becomes a digital signal.


Will anybody explain me what actually it means?
This sounds like a confusion in terminology. See http://en.wikipedia.org/wiki/Digitizing

Digitization basically involves two steps: 


Discretization: Sampling the signal at discrete times
Quantization: Turning the samples from (in theory) infinite resolution to finite resolution


Look at all possible combinations you can have 4 different types of signal:


continuous time, continous amplitude
discrete time, continous amplitude
continuous time, discrete amplitude
discrete time, discrete amplitude


Typically we refer to #1 as "analog" and #4 as "digital". #2 and #3 are of little practical relevance, other than getting the mathematical models right. Quantization and Discretization are really quite different mathematically so it doesn't make a lot of sense to analyze them in combination. In other words: the theory of time discretization applies equally to continuous amplitude and discrete amplitude signals.