Short Answer
Narrow-Body Aircraft – One aisle
Wide-Body Aircraft – More than one aisles (generally, two)
Long Answer
The distinction between Narrow- and Wide-Body aircraft is the width of the fuselage (tube shaped aircraft body, or the part where passengers, cargo and crew sit).
A typical narrow-body plane has a diameter of 3-4 m whereas a typical wide-body plane has a diameter of 5-6 m.
Due to the bigger fuselage, wide-body aircraft can house more passengers, and the same is achieved by having more seats per row. While narrow-body aircraft will have 3-6 seats per row with one aisle, wide-body aircraft can have upto 10 seats with two aisles.

Credits: Altair78

Credits: Badudoy
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Definitions
International Civil Aviation Organisation (ICAO) gives the following definitions for the types:
- A wide-body aircraft is a large transport aircraft with internal cabin width sufficient for normal passenger seating to be divided into three axial groups by two aisles (in practice this means not less than 4.72 metres (15.6 feet))
- A narrow-body aircraft is an aircraft having only one aisle in the cabin with passenger seating divided into two axial groups.
Examples
Head over to these articles to know which of the important aircraft flying in the skies today are narrow- and wide-body.
Providing some notable examples below:
Largest Narrow-Body Aircraft
Boeing 757-300

Credits: Cory W. Watts
Smallest Wide-Body Aircraft
Airbus A310

Credits: Tucancillo
First wide-body aircraft developed
Boeing 747

Credits: Robert Frola
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