Along with the classification of signals below, it is also important to understand the Classification of Systems.
ContinuousTime vs. DiscreteTime:
As the names suggest, this classification is determined by whether or not the time axis (xaxis) is discrete (countable) or continuous (Figure 1). A continuoustime signal will contain a value for all real numbers along the time axis. In contrast to this, a discretetime signal is often created by using the sampling theorem to sample a continuous signal, so it will only have values at equally spaced intervals along the time axis.
Analog vs. Digital:
ContinuousTime vs. DiscreteTime:
As the names suggest, this classification is determined by whether or not the time axis (xaxis) is discrete (countable) or continuous (Figure 1). A continuoustime signal will contain a value for all real numbers along the time axis. In contrast to this, a discretetime signal is often created by using the sampling theorem to sample a continuous signal, so it will only have values at equally spaced intervals along the time axis.
Figure 1 
The difference between analog and digital is similar to the difference between continuoustime and discretetime. In this case, however, the difference is with respect to the value of the function (yaxis) (Figure 2). Analog corresponds to a continuous yaxis, while digital corresponds to a discrete yaxis. An easy example of a digital signal is a binary sequence, where the values of the function can only be one or zero.
Figure 2 
Periodic vs. Aperiodic:
Periodic signals repeat with some period T, while aperiodic, or nonperiodic, signals do not (Figure 3). We can define a periodic function through the following mathematical expression, where t can be any number and T is a positive constant:
f(t) = f(T + t).........(1)
The fundamental period of our function, f(t) is the smallest value of T that the still allows Equation (1) to be true.
Periodic signals repeat with some period T, while aperiodic, or nonperiodic, signals do not (Figure 3). We can define a periodic function through the following mathematical expression, where t can be any number and T is a positive constant:
f(t) = f(T + t).........(1)
The fundamental period of our function, f(t) is the smallest value of T that the still allows Equation (1) to be true.
(a) A periodic signal with period To 
(b) An aperiodic signal
Figure 3

Causal vs. Anticausal vs. Noncausal:
Causal signals are signals that are zero for all negative time, while anticausal are signals that are zero for all positive time. Noncausal signals are signals that have nonzero values in both positive and negative time (Figure 4).
Causal signals are signals that are zero for all negative time, while anticausal are signals that are zero for all positive time. Noncausal signals are signals that have nonzero values in both positive and negative time (Figure 4).
(a) A causal signal 
(b) An anticausal signal 
(c) A noncausal signal
Figure 4

Even vs. Odd:
An even signal is any signal f such that f(t)=f(t). Even signals can be easily spotted as they are symmetric around the vertical axis. An odd signal, on the other hand, is a signal f such that f(t)=(f(t)).(Figure 5).
An even signal is any signal f such that f(t)=f(t). Even signals can be easily spotted as they are symmetric around the vertical axis. An odd signal, on the other hand, is a signal f such that f(t)=(f(t)).(Figure 5).
(a) An even signal 
(b) An odd signal
Figure 5

Using the definitions of even and odd signals, we can show that any signal can be written as a combination of an even and odd signal. That is, every signal has an oddeven decomposition. To demonstrate this, we have to look no further than a single equation.
f(t)=1/2(f(t)+f(t))+1/2(f(t)−f(t)).............(2)
By multiplying and adding this expression out, it can be shown to be true. Also, it can be shown that f(t)+f(t) fulfills the requirement of an even function, while f(t)−f(t) fulfills the requirement of an odd function (Figure 6).
By multiplying and adding this expression out, it can be shown to be true. Also, it can be shown that f(t)+f(t) fulfills the requirement of an even function, while f(t)−f(t) fulfills the requirement of an odd function (Figure 6).
(a) The signal we will decompose using oddeven decomposition 
(b) Even part: e(t)=1/2(f(t)+f(t)) 
(c) Odd part: o(t)=1/2(f(t)−f(t))
Figure 6

Deterministic vs. Random:
A deterministic signal is a signal in which each value of the signal is fixed and can be determined by a mathematical expression, rule, or table. Because of this the future values of the signal can be calculated from past values with complete confidence. On the other hand, a random signal has a lot of uncertainty about its behavior. The future values of a random signal cannot be accurately predicted and can usually only be guessed based on the averages of sets of signals (Figure 7).
A deterministic signal is a signal in which each value of the signal is fixed and can be determined by a mathematical expression, rule, or table. Because of this the future values of the signal can be calculated from past values with complete confidence. On the other hand, a random signal has a lot of uncertainty about its behavior. The future values of a random signal cannot be accurately predicted and can usually only be guessed based on the averages of sets of signals (Figure 7).
(a) Deterministic Signal 
(b) Random Signal
Figure 7

RightHanded vs. LeftHand Signal:
A righthanded signal and lefthanded signal are those signals whose value is zero between a given variable and positive or negative infinity. Mathematically speaking, a righthanded signal is defined as any signal where f(t)=0 for
A righthanded signal and lefthanded signal are those signals whose value is zero between a given variable and positive or negative infinity. Mathematically speaking, a righthanded signal is defined as any signal where f(t)=0 for
(a) Righthanded signal 
Finite vs. Infinite Length:
As the name applies, signals can be characterized as to whether they have a finite or infinite length set of values. Most finite length signals are used when dealing with discretetime signals or a given sequence of values. Mathematically speaking, f(t) is a finitelength signal if it is nonzero over a finite interval
>t_{1} <>2 Where t_{1} >  α and t_{2} < α .An example can be seen in Figure 9. Similarly, an infinitelength signal f(t) is defined as nonzero over all real numbers: α ≤ f(t) ≤  α As the name applies, signals can be characterized as to whether they have a finite or infinite length set of values. Most finite length signals are used when dealing with discretetime signals or a given sequence of values. Mathematically speaking, f(t) is a finitelength signal if it is nonzero over a finite interval
Figure 9: FiniteLength Signal. 
Note that it only has nonzero values on a set, finite interval.