Ignorance in Buddhism is not knowing the the Four Noble Truths.It is not knowing the truth behind suffering.

"And what is ignorance,...Not knowing about dukkha. MN 9 (Ñanamoli/Bodhi, trans.). Avijja, the Pali word for ignorance, is the opposite of vijja, which means not only "knowledge" but also "skill" — as in the skills of a doctor or animal-trainer. So when the Buddha focuses on the ignorance that causes stress and suffering, saying that people suffer from not knowing the four noble truths,... Ignorance by Thanissaro Bhikku

Ignorance is similar to delusion but with subtle differences.In order to find the difference between delusion and ignorance i will first provide a definition for 'delusion'.

a belief that is not true : a false idea Merriam-webster dictionary

Comparing this definition with 'ignorance' in the buddhist context,delusion is a view based on ignorance.Delusion is similar to wrong view and ignorance is the not knowing itself.

To give a real world example,a person drives a car badly but think he is driving a car well.The person is deluded.Why is the person deluded?Because the person doesn't know what is good driving and bad driving.(Ignorance).The lack of knowledge in itself is ignorance and the actions that stem from it is delusional.