Is Lecture Notes by Springer a journal?
The Springer Lecture Notes in Computer Science (LNCS) series (and its related sub-series) are rather odd publications. The DBLP bibliography server marks them as a journal in one way, but categorizes them as a series in another way, and they are structured and reviewed more like conference proceedings or collections of book chapters.
I personally would not think of them as being the equivalent of a journal publication. Whether they count as a journal publication for a particular organization's set of regulations, however, is something that only that organization can determine.
I think the technical answer is no. Rather it is a series of research monographs.
Clearly there is a continuum here, and actually the SLN[X] series seems to have become more journal-like since the last time I checked. (The SLNM webpage lists an inmpact factor, for instance.) I think you can do no better than to consult the series homepages:
Springer Lecture Notes in Computer Science.
Springer Lecture Notes in Mathematics.
Springer Lecture Notes in Physics.
In CS the series seems to be organized into many subseries, however each volume gets a "global number".
However in all three cases you can check the language used and see that they talk about "monographs", "titles" and "texts", never "journals". If my memory is accurate, librarians view them this way as well: sometimes journals can be checked out for a much shorter time than books, and in my experience the SLNM have always been treated like books.
For the math series, from the linked page you can click to get a four page pdf file detailing the editorial policy for the productions of the LNM monographs. The following passage seems rather enlightening:
Monograph manuscripts should be reasonably self-contained and rounded off. Thus they may, and often will, present not only results of the author but also related work by other people. They may be based on specialised lecture courses. Furthermore, the manuscripts should provide sufficient motivation, examples and applications. This clearly distinguishes Lecture Notes from journal articles or technical reports which normally are very concise. Articles intended for a journal but too long to be accepted by most journals, usually do not have this "lecture notes" character. For similar reasons it is unusual for doctoral theses to be accepted for the Lecture Notes series, though habilitation theses may be appropriate.
I could not find the analogous file for either the SLNCS or SLNP.
In terms of the specific question:
Some universities require the PhD students to publish in journals in order to graduate, so I want to know. I am in Computer Science field.
This is a question about academic culture, both general CS culture and the culture of your specific department and university. You certainly need to ask people in your own local culture. As you can see above, in mathematics graduate students rarely publish in SLNM: PhD theses are generally not appropriate, and it is hard to see what other book-length partially expository high level research document it would be worth the time of a graduate student to write and publish. But it looks like CS does things a bit differently...
Status of LNCS is rather strange, but it is surely not a journal. Each book is a proceeding (I'm not aware of any exceptions to this rule), and altogether it's simply a series. I'll try to explain why:
Note that while it has an ISSN, each of its volumes gets an ISBN. This makes it a "serial" publication, but each of the books is ... well, a "book". (Journal issues do not get ISBN.)
Springer itself lists most (if not all) of the books as proceedings, including the dates of the conference etc. The conference dates are usually not listed for special issues for instance, it's most often only mentioned in the preamble. (Jounal issues don't get "names".)
You get invited talks listed there which are not peer-reviewed. (In journals, only announcements are -- in general -- not peer-reviewed.)
It has no periodicity. (Journals have to be periodical; this periodicity can change over time, and supplements can appear that break the periodicity, but if LNCS were a journal, it would be a journal with no regular issues published periodically.)
It does not have, and cannot have by current standards, impact factor assigned. (Journals can have IF, obviously.)
To conclude: Each book is a proceeding, and the papers are "inproceedings" by BibTeX standard. Note that according to many bibliography styles, you should list the name of the proceeding together with its editors, and the volume number in the series. I speak about LNCS, but it's the same for all Springers series, and not only them, the same applies for instance to EPTCS as well.