You have to take some time to read the classic "Manual of Osteosynthesis" by authors (Mueller et al.) from the Arbeitsgemeinschaft fuer Osteosynthesefragen. It underwent many editions since the 70,s. Anyway, it should give you an essential answer, although per se it is an orthopedic surgery manual. 

In short, the so-called positioning screws implement only fixation between the fragments, while the lag screws realize fixation and compression between the fragments to enhance osteotomy (fracture) healing.

The lag screw principle is based on making drill hole through the 2 bone fragments that needs to be compressed together then the drill hole through the nearby fragment is widened to be equal to the outer diameter of the lag screw so that the screw passes through the nearby fragment without engaging into it & at the same time it will engage through the far fragment &  with tightening  of the screw the head of the screw will push the nearby fragment toward the far fragment which is pulled by the screw threads  leading to compression at the fracture or osteotomy plan unlike the positioning screw which hold the 2 fragment together without compression unless the 2 fragments are compresses together by another mechanical measure.

https://www.youtube.com/watch?v=6riMX7Mg0DQ

https://www.youtube.com/watch?v=32fAdOZVxXI

@Dr. Bassem: I agree with your description of the lag screw mechanism of action. This is the typical case. However, I have to disagree with the statement that once in place the fixed position of the fragments (using positioning screws) may be influenced by another mechanical measure to add compression. 

What I mean by other mechanical measure is to do the compression before inserting the positioning screw which will maintain the compression not to induce it as for example when we use bone holding clamp to compress a fracture or using the compression princople of the dcp plate or prebending of plate or when using tension device.

Thank you, Dr. Bassem. Yes, compression before insertion of positioning screws is possible. Not vice versa. 

Regards

according to biomechanics lag screw(inter fragmentary fixation) applied for compression between bone fragments like oblique fracture, spiral fracture, or butterfly fragment but positioning  screw used for just maintaining  the position of bone fragment without compression because compression not needed in such situation because it will lead to bone collapse and decrease in bone geometry

Ramus osteotomy performed in the mandible usually for elongation and requires a long split. Lag screws are those which will compress the split "fragments" of the osteotomy and promote "primary" healing. They as described have the principle that the lateral hole has larger diameter than the medial. This way as the screw is advanced the lateral "fragment" is driven on to the medial and the two are compressed together. They are placed perpendicular to the osteotomy line. After this positional screws are placed (for these the holes in both fragments have the same diameter. They are placed to stabilised and re-enforce the osteotomy and the fixation. In some cases plates are used or even wires. AO principles or internal fixation.