Admittance:

Conductance and susceptance combine to form the admittance, denoted by capital letter Y. Admittance, in an alternating current circuit, is analogous to conductance in a direct current circuit.

Complex admittance

Admittance is a complex quantity and represents ease with which the current can flow in an alternating current circuit. As the absolute value of the impedance gets larger, absolute value of admittance becomes smaller. Huge impedances correspond to tiny admittances, and vice-versa of it.

Admittances are written in the complex form like impedances.  But you are required to keep track of which quantity you are talking about! This will be obvious if you use symbol, like Y=3 - j0.5 or Y =7+j3. When you see Y rather than Z, you know that negative j factors (like -j0.5) mean that there is a average inductance in the circuit, and positive j factors (like+j3) mean there is net capacitance.

Admittance is complex composite of the conductance  and susceptance. Therefore, the admittance takes the form

Y = G +jB

The j factor can be negative, of course, so there are times you will write Y = G -jB.

Parallel circuits

You saw series RL and RC circuits. You wondered why parallel circuits were not given importance in those discussions. The reason is that admittance, instead of impedance, is best for working with the parallel alternating current circuits. Thus, the subject of parallel circuits was deferred.

Resistance and reactance combine in messy manner in parallel circuits, and it can be hard to envision what is happening. But conductance (G) and susceptance (B) simply add together in parallel circuits, yielding admittance (Y). This simplifies the analysis of parallel alternating current circuits.

The situation is just like the behavior of resistances in parallel when you work with direct current. While the formula is a little cumbersome if you are required to find the value of a bunch of resistances in the parallel, it is easy to simply add the conductances.Now, with alternating current, you are working in two dimensions rather than one. That is the only difference between them.