By 'Introduction to Philosophy', I mean at least a textbook (of at least 400 pages) for an undergraduate genuinely interested in philosophy but lacking any prerequisite, that (at least) isolates, bolds and defines key philosophical terms; and explains introductory concepts simply.
In general, the "histories" or "anthologies" present exemplary works by notable philosophers, who are usually innovators in the field. It is like reading Einstein's own popular writings as opposed to synoptic works about Einstein's theories. A preface may provide enough to set the works in context and relate them.
The "introductory textbooks" are usually written by academics who overview the notable philosophers in a transitional and presumably "objective" way. Typically, they may be commissioned by publishing companies to fit course requirements and attempt an "even-handed" approach to philosophers who may have differed passionately.
Strangely, one often finds that in science and philosophy, works on theory by the ones who originated those theories are actually clearer than the secondary "clarifications." But not always.
Primary texts, even excerpted in anthologies, should never be avoided and are usually enlightening, but also have a snobbish appeal. As someone who has only read philosophy on my own, I am a defender of synopses, introductions, and secondary literature. Quality varies. But they are good ways to enter into the complex thinking of the past or present. And some are gems of reduction and insight.
This is mostly an English question rather than a philosophy question, but it might have enough merit to be worth including in philosophy.
In general, an anthology in philosophy collects articles either on a topic or from a period. These volumes serve two purposes. For undergraduate teaching, they collect together famous bits -- Aquinas's Five Ways, Descartes' Wax Argument and and/or Cogito, Kant's "What is Enlightenment?", Quine's "Two Dogmas of Empiricism", etc. Or alternately, a bunch of articles on a single topic.
a history of philosophy often is the author writing about what happened in a historical period within philosophy. Thus, they might talk about the role of Descartes' cogito and the foundationalist project shared by the empiricists and the rationalists. It may quote but probably is not composed of primary sources.
An introduction is usually topical. In this sense, it's like the history for a topic rather than a period. Again, this is usually the work of a single author rather than the original sources.