Moodle has a number of core APIs that provide tools for Moodle scripts.API's are already having functionality that can be reuse,and allow you to interact with Moodle system. They are essential when writing Moodle plugin, or Moodle customization.
Moodle Questions, those are about programming or development in Moodle should use this tag.
there are many core-apis those provide interaction with Moodle core system.
List of API's is
1 Most-used General APIs
1.1 Access API (access)
1.2 Data manipulation API (dml)
1.3 File API (files)
1.4 Form API (form)
1.5 Logging API (log)
1.6 Navigation API (navigation)
1.7 Page API (page)
1.8 Output API (output)
1.9 String API (string)
1.10 Upgrade API (upgrade)
1.11 Moodlelib API (core)
2 Other General APIs
2.1 Admin settings (admin)
2.2 Availability (availability)
2.3 Backup API (backup)
2.4 Cache API (cache)
2.5 Calendar API (calendar)
2.6 Comment API (comment)
2.7 Data definition API (ddl)
2.8 Enrolment API (enrol)
2.9 Events API (event)
2.10 External functions API (external)
2.11 Lock API (lock)
2.12 Message API (message)
2.13 Media API (media)
2.14 My profile API
2.15 Preference API (preference)
2.16 Portfolio API (portfolio)
2.17 Rating API (rating)
2.18 RSS API (rss)
2.19 Tag API (tag)
2.20 Task API (task)
2.21 Time API (time)
2.22 Testing API (test)
2.23 User-related APIs (user)
2.24 Web services API (webservice)
3 Activity module APIs
3.1 Activity completion API (completion)
3.2 Advanced grading API (grading)
3.3 Conditional activities API (condition) - deprecated in 2.7
3.4 Groups API (group)
3.5 Gradebook API (grade)
3.6 Plagiarism API (plagiarism)
3.7 Question API (question)
more about Moodle Api's https://docs.moodle.org/dev/Core_APIs
Most generally used api's are
Form API, File API, Events Api, Logging API, Data manipulation API, Navigation API (navigation) etc.
