Small Signal Model - SubThreshold.com

Parameters

g_m: Gain factor for v_gs. Or the change in drain current (i_D) at a given Q point $g_m = \frac{\del i_D}{\del v_{GS}} = \frac{ 2 I_D }{ v_{GS} - v_{TH} } = sqrt{ 2 \mu_n C_{ox} \frac{W}L I_D }$
g_ds: Channel Conductance - Small signal drain to source admittance. This is due to channel length modulation. Or the change in drain to source voltage (V_DS) at a given Q point $g_{ds} = \frac{\del I_D}{\del V_{DS}} = \lambda \frac1{2} \mu_n C_ox \frac{W}{L} (V_{GS} - V_{TH})^2 \approx \lambda I_D$ The inverse, $1/g_{ds} = r_o$ is output resistance.
g_s: Body effect gain factor for v_s. Or the change in drain current (i_D) at a given Q point $g_s = \frac{\del i_D}{\del v_{SB}} = \lambda I_D = \frac{\gamma g_m}{2 sqrt{V_{SB} + \left| 2 \phi_F \right| }}$

For a single transistor we can say: $i_D = g_m v_{G} + g_d v_{D} - (g_m + g_d) v_S$ (See Rules Of Thumb for interpretation)