What are the main difference between Internet-Based Testing (iBT) and Paper based Testing(PBT) formats?

TOEFL iBT measures reading, listening, speaking and writing skills and takes approximately 4½ hours. It is given on computer 30-40 times a year at test centers throughout the world.

TOEFL PBT measures listening, structure (grammar), reading and writing and takes approximately 3½ hours. It is given using paper and pencil 6 times a year at test centers where TOEFL iBT is not available. 

Section	Internet Based Test (IBT)		Paper-Based Test (PBT)
Reading	3-5 passages from academic texts; approximately 700 words long; 12-14 questions per passage. What's different? Glossary to define key words; easy-to-use review feature New task – categorize information and fill in chart/complete summary Time:60-100 minutes Questions:36-70 Score Scale:0-30		measures the ability to understand non-technical reading material. Time:30 minutes Questions:1 topic Score Scale:31-67
Listening	4-6 lectures, some with classroom discussion; each 3-5 minutes long; 6 questions each. 2-3 conversations; each 3 minutes long; 5 questions each. Time:60-90 minutes Questions:34-51 Score Scale:0-30 10 minutes break What's different? Longer, more authentic conversations and lectures Note taking allowed New questions that measure understanding of a speaker's attitude, degree of certainty, purpose, or motivation		Listening Comprehension measures the ability to understand English as it is spoken in North America. 3 – 4 mini-talks: each 60 – 90 seconds long, 3 – 5 questions each 2 – 3 extended conversations: each 60 – 90 seconds long, 3 – 5 questions each 30 – 40 dialogues: each 5 – 15 seconds long, 1 question each Time:30-40 minutes Questions:50 Score Scale:31-68
Speaking	2 tasks to express an opinion on a familiar topic; 4 tasks to speak based on what is read and listened to. Up to 30 seconds to prepare the response Up to 1 minute to respond Time:20 minutes Questions:6 tasks Score Scale:0-4 points converted to 0-30		No Speaking Section
Structure (grammar)	No Structure Section No grammar section, although grammar is evaluated in Speaking and Writing responses.		This part has 15 exercises of completing sentences correctly and 25 exercises of identifying errors. Time:25 minutes Score Scale:31-68
Writing	1 task to write based on what is read and listened to; 1 task to support an opinion on a topic. Time:50 minutes Questions:2 tasks Score Scale:0-5 points converted to 0-30		One essay with 250-300 words in average. Time:30 minutes Questions:1 topic Score Scale:0-6
Total score	Score Scale:0-120		Score Scale:310-677

Source : https://www.interface.edu.pk/tests/toefl/toefl-test-formats.asp