I want to know the Type declaration and signal declaration.

It allows you to define new data types that allow you to write code that is easier to understand.

For example, lets say I am writing some DSP code using the VHDL data type "signed", but I have an array of signals, eg., the inputs to a parallel FFT, and I want to write a loop ... Since VHDL does not have an array of signed data type, I can define one ...

type signed_vector is array(natural range <>) of signed;
-- An Array of 8 16-bit signed values
signal fft_input  : signed_vector(0 to 7)(15 downto 0);
signal fft_output : signed_vector(0 to 7)(15 downto 0);
-- Some DSP logic ... perhaps involving loops, eg.,
for i in 0 to 7 loop
    fft_output(i) <= fft_input(i);
end loop;

Cheers,

Dave