UGC NET & JRF are same exams. The only difference is that you need more marks for JRF as compared to NET. Top 6% of students who qualify NET exam gets JRF.
NTA Conducts the UGC NET exam twice a year. In this exam there are two parts: (1) NET (Lecturership)- No age limit to apply (2) JRF (Junior Research Fellowship)- Maximum age limit is 30 years
If you clear the NET then your UGC NET certificate is valid for life time. After Qualifying NET a person is eligible to apply for the position of Lecturer/Assistant Professor in Universities and Colleges located anywhere across India
When you qualify for Jrf which is generally 5% to 7% more than that the cut off of NET, you will be eligible for JRF where you will get fellowship (approx. 25,000 per month) from government while you do your PhD. If you qualify JRF, then your NET is automatically cleared.
You can become assistant professor after clearing NET as well as JRF. The only difference between these two are that in JRF you get monetary scholarship and in lectureship, you do not get scholarship.
In short, you can do Ph.D with scholarship after clearing JRF.