ज

We would use J of English for romanization. This letter is pronounced as the sound of J in English. The sound of J in Jail in English is same as ज in Hindi Language.  Some more example from English are

J as in Jam

J as in Jar

जग is read as Jag (Pronounced as Jug in English)  in Hindi which means same in Hindi as in English.

Hindi Examples

जल Jal Water

जब Jab When

झ

When this letter is pronounced there is more air being released from mouth so it is a aspirated version of ज. There is no exact sound of झ is present in English. But the sound of SU in the word Measure in English can be said to be closely related. It has been described just to give an familiarity with English. However, it is not at all same as झ of Hindi Language. It would be represented as Jhh.

For Example

झटपट Jhhatpata hurry

झलक Jhhalaka  A glance