what things are duty free?

Different countries have different rules.
Here is the extract of rules for Australia:
Duty-free concessions in Australia are different to those in other countries.

Most personal items such as new clothing, footwear, and articles for personal hygiene and grooming (excluding fur and perfume concentrates) may be brought into Australia in your accompanied baggage, free from duty and tax.

Personal goods are free from duty and tax if they are:

* owned and used by you overseas for 12 months or more
* imported temporarily (a security may be required by Customs)

For other goods, limits apply. These include goods that are purchased overseas and goods that are purchased in Australia duty or tax free (that have been previously exported), or from an inwards duty free shop on arrival into Australia. Also included are goods for which a TRS claim has been made.
Duty free concessions do not apply to commercial goods.

General goods
If you are aged 18 years or over, you can bring up to A$900 worth of general goods into Australia duty-free. If you are under 18 years of age there is a A$450 limit. General goods include gifts, souvenirs, cameras, electronic equipment, leather goods, perfume concentrates, jewellery, watches and sporting equipment.

Alcohol
If you are aged 18 years or over, you can bring 2.25 litres of alcohol duty-free into Australia with you. All alcohol in accompanied baggage is included in this category, regardless of where or how it was purchased.

Tobacco
If you are aged 18 years or over, you can bring 250 cigarettes, or 250 grams of cigars or tobacco products duty-free into Australia with you. All tobacco products in accompanied baggage are included in this category, regardless of where or how they were purchased.