Is a conference paper whose proceedings will be published in IEEE Xplore counted as a publication?

I would be a bit hesitant to include it as a "publication" rather than as "under review". I don't know what field or conference you are referring to, but in CS, my experience is that review happens before the conference (and before acceptance) rather than after. And since in CS, conferences are a main publication venue, it would seem appropriate to include under those circumstances. You seem to be describing something different, however.

But your words "will be published...after the papers have been reviewed" makes this seem a bit tentative.

I suggest you wait before you include it as a publication unless you qualify it in some way. You don't need to omit it entirely, just be honest and clear about the actual state of the world when you send it out.

Can I include it in my CV under publications? Does this count as an electronic publication since it's going to be on IEEE Xplore? Or should I write it under conferences? This is my one and only paper.

It sounds like your paper has been accepted for publication, so yes, you can list it in your CV as a publication (assuming your research community accepts refereed conference papers as publications).

Even though you've already submitted the final camera-ready version, the proceedings has to work its way through IEEE's publication pipeline: assembling the table of contents, adding page numbers, possibly checking for compliance with formatting instructions, extracting and indexing metadata, creating cross-reference links for the bibliography, printing and shipping paper copies (if they still do that), and so on. The conference organizers should be able to tell you when they expect the proceedings to appear.

Since the paper isn't out yet, I heard that forthcoming/in review/in press,etc must be used on the CV. Which term should I use here since my paper will probably not count as "in press" material? Also, what format must I use in my CV to mention this paper?

I would list it as either "to appear" or "in press", but the precise format doesn't really matter. Just be clear.

I read that depending on the area, a conference paper has much value or no value at all and in some fields like Computer Science, a conference paper has good value. Is this a fact?

Yes, this is a fact. In most (but not all!) fields of computer science, conference proceedings are the primary venue for publishing research, not journals.

What are "refereed" conference proceedings? Do IEEE proceedings fall into this category?

Refereed conferences (which are fairly typical in CS) typically have a "program" or "papers" committee, which is charged with reviewing submitted manuscripts and deciding which to accept for publication in the proceedings and presentation at the conference (either as a talk or a poster or both). Typically, every presented paper appears in the proceedings, every paper in the proceedings is presented, and the proceedings is published (slightly) before the conference begins. The submission itself is either a full paper or an extended abstract, typically between 6 and 12 pages long (depending on the conference), not just a single-page abstract. The initial submission deadline is several months before the conference, to give the committee enough time to gather/write multiple reviews for each paper, choose which papers to accept, have authors revise their papers and prepare the camera-ready versions, and actually publish the proceedings. (The publisher is only involved in the very last step.) Conference reviews are exactly analogous to referee reports for journals, and for many conferences just as thorough and detailed.

Of course, details vary significantly from one conference to the next. The details for your conference should have been spelled out in complete detail in the call for submissions.

Whether a given conference proceedings is refereed (and if so, how stringently) is up to the conference organizers. You can't tell from the publisher. Most proceedings published by IEEE are refereed, but not all.

IEEE Xplore

I looked up IEEE Xplore and saw that it is a digital library, not a journal [1]:

The IEEE Xplore® digital library provides access to IEEE journals, transactions, letters, magazines and conference proceedings, IET journals and conference proceedings, IEEE Standards and IEEE educational courses.

Therefore, it does not count as a publication beyond your original conference publication. It currently contains over 4 million items [2].

Conferences and Journals in CS/CE

That said, an IEEE conference publication should not be considered unimportant due to not being in a journal. In some fields of computer science and engineering, conference publication is the gold standard. The Computing Research Association published a best practices memo, Evaluating Computer Scientists and Engineers For Promotion and Tenure written by 3 senior computer scientists at top departments: David Patterson (who went on to win the Turing Award), Larry Snyder, and Jeffrey Ullman. The first paragraph reads:

The evaluation of computer science and engineering faculty for promotion and tenure has generally followed the dictate “publish or perish,” where “publish” has had its standard academic meaning of “publish in archival journals” [Academic Careers, 94]. Relying on journal publications as the sole demonstration of scholarly achievement, especially counting such publications to determine whether they exceed a prescribed threshold, ignores significant evidence of accomplishment in computer science and engineering. For example, conference publication is preferred in the field, and computational artifacts —software, chips, etc. —are a tangible means of conveying ideas and insight. Obligating faculty to be evaluated by this traditional standard handicaps their careers, and indirectly harms the field. This document describes appropriate evidence of academic achievement in computer science and engineering.

My colleagues and I used to include that memo in our promotion and tenure dossiers, to positive effect.

That said, not all conferences are created equal. You should research and document the ranking of the conference that published your paper. The CORE Rankings Portal is a good place to start if your area is computing.