Signals updates a value after some "delta" time or at the
end of the process. But variable updates a value immediately.

Both variable and signals are synthesizable. 
Designer should know hoe to use these 2 objects.

Ex: Signal usage
Library IEEE;
use IEEE.std_logic_1164.all;
entity xor_sig is
port (
A, B, C: in STD_LOGIC;
X, Y: out STD_LOGIC
);
end xor_sig;
architecture SIG_ARCH of xor_sig is
signal D: STD_LOGIC;
begin
SIG:process (A,B,C)
begin
D <= A; -- ignored !!
X <= C xor D;
D <= B; -- overrides !!
Y <= C xor D;
end process;
end SIG_ARCH;

Variable usage:
Library IEEE;
use IEEE.std_logic_1164.all;
use IEEE.std_logic_unsigned.all;
entity xor_var is
port (
A, B, C: in STD_LOGIC;
X, Y: out STD_LOGIC
);
end xor_var;
architecture VAR_ARCH of xor_var is
begin
VAR:process (A,B,C)
variable D: STD_LOGIC;
begin
D := A;
X <= C xor D;
D := B;
Y <= C xor D;
end process;
end VAR_ARCH;