What is Ideal Transformer ?
An ideal transformer is one that has no losses i.e., its windings have no resistance, there is no magnetic leakage, and hence which has no iron and copper losses. It may, however, be noted that it is impossible to imagine such a transformer in practice, yet for convenience, we have to assume such a transformer.
Ideal Transformer Model :
Let us consider an ideal transformer as shown in the figure, whose primary is connected to sinusoidal alternating voltage V_{1} and whose secondary is open. This V_{2} causes an alternating current to flow in the primary.
Since the primary coil is purely inductive and the secondary being open ( no output ) the primary draws the magnetizing current I_{µ} only. The function of this current is merely to magnetize the core, it is small in magnitude and lags V_{1} by 90°. This current I_{µ} produces an alternating flux 'φ' which is, at all times proportional to the current and hence, is in phase with it as shown in the phasor diagram. This flux φ links with both primary and secondary windings.
Therefore, it produces selfinduced e.m.f. E_{1} in the primary. This E_{1} is at every instant equal and opposite to V_{1}. It is also known as counter e.m.f. or back e.m.f. of the primary.
Similarly, e.m.f. E_{2} is induced in the secondary and is known as mutually induced e.m.f due to E_{1}. This E_{2} is in antiphase with V_{1} ( and is proportional to the number of turns in the secondary ).
Check the below video for more detailed explanation,
Difference Between Ideal and Practical Transformer ( nonideal transformer ) :
Ideal Transformer 
Practical Transformer 

1. Permeability of an ideal transformer is always infinity, i.e., µ = ∞. 
1. There exists an finite permeability in practical transformer, i.e., µ ≠ ∞. 
2. In an ideal transformer the core losses i.e., lron losses is equal to zero. 
2. The iron losses of an practical transformer is given I_{w. }

3. The winding resistance of an ideal transformer, i.e., R = 0. Hence, copper losses in the windings of the transformer are absent. 
3. In practical the winding resistance cannot be zero in a transformer, i.e., R ≠ 0. The total resistance when reffered to the primary side of a transformer is given by R_{01} = R_{1} + R_{2} / K^{2}. 
4. In an ideal transformer flux produced by the primary winding will link with the secondary winding without any leakage of flux. 
4. In case of practical transformer there will be a wastage of flux due to leakage ( not linking total flux produced by primary with secondary ). 
5. It is impossible to make an ideal transformer. All the transformer made will consists of losses in its. 
5. The transformers which we see in our daily life are of practical transformers. 