Normally in Thailand each academic year there are two semesters. The first semester starts in the middle of May and ends around early of October, and the second semester starts early of November to end of March. So, Thai students generally have 2 school holiday periods which are October and April plus first half of May. No half semester break, but students would have days off during semesters on public holidays.
Less crowded and traffic
If you travel in Bangkok by BTS Skytrain or MRT Subway, it is not that obvious. You might just see less crowded while you travel. But, these two periods are like a heaven for office workers who drive and work in Bangkok. The traffic is less than normal. An-hour drive from home to work place could become only 15-minute drive. And, you can expect to see very less traffic on public holidays because they are days off for both students and office workers.
Are these two periods good for tourists?
Of course, but not that much. It is a great time to visit Bangkok, if you plan to drive. Anyway, I do not recommend you to explore this city by driving. Public transportation modes such as BTS Skytrain and MRT Subway are MUCH better!
Hope this information helps!