What type of charge is produced when an electric field separates positive and negative charges?
In a way, you have answered your own question! All objects that have an electric charge at all have a charge which is either positive or negative. In either case, the charge can be large or small. The charge of the object has a particular value corresponding to a positive number for positive charges and a negative number for negative charges. Objects with no charge, or neutral objects, can be thought of as having an electric charge of zero. So it is easiest to think of the charge of an object as a number of charge units, where that number can be positive, negative or zero.
So let's ask a slightly different version of your question: I've heard of positive and negative charges separating in an electric field. What is an example of this happening? Here is an example: A neutral atom of gas, like argon, is sitting in an electric field, and one of its electrons gets knocked off by a charged particle which comes flying by very close to it. The flying charged particle continues on, leaving the knocked off electron behind in the electric field. Now, the argon atom has been separated into two pieces: an argon ion with positive charge, +1 unit, and the knocked-off electron with negative charge, -1 unit. These two oppositely charged objects will separate further in the electric field if that field is strong enough. In fact, several of the particle detectors at Jefferson Lab work via this exact physical process.
Mark Ito, Staff Scientist (Other answers by Mark Ito)