As explained here on the ITF Tennis website, you're pretty much right. There's two different "types" of grades, A, B, and C, as well as 1-5. Grades A, B, and C represent different types of competitions as follows:
- Grade A: Top-tier tournaments - four Grand Slams plus 5 other events
- Grade B: Regional Championships
- Grade C: International Team Competitions
Grades 1-5 are basically the same as the ATP/WTA GS/1000/500/250 system - a Grade 5 event is worth the least points, and a Grade 1 event is worth the most.
Grade A events are always worth more than Grade 1 events, but Grade B events are generally denoted with something like "B2", which means that it's a regional championship worth slightly more than a Grade 2 event. Similarly, B1 events are worth slightly more than Grade 1s (but less than Grade As), and B3 events are worth more than Grade 3s, but less than Grade 2s. There are currently no Grade B events lower than B3.
To be on the end-of-year rankings a junior player must have played in three Grade A events.