Please help us to understand what is methodology and what is guide.

I assume you are talking about general (Project Management) Methodology, and PMBOK Guide. Let's take methodology first. A methodology is "... a system of practices, techniques, procedures and rules used by those who work in a discipline." Since Project Management, as we study it, has these items, we can call it a methodology. Guide is not so formally defined; let's use "... something that advises or shows the way..." The PMBOK Guide shows the way/advises in Project Management Good Practice. Now, this is an interesting side-note. The PMBOK is actually a Guide and Standard. The PMBOK Guide, which starts on page 1 of the PMBOK, is the advisory part. It shows you the way to 'do' Project Management. Another section is the Standard for Project Management - this starts on p. 541. This is what is aligned with ISO 21500. The Guide references the standard, and gives more details as well as great context.