Sunauli (actually "Bhairawa" or "Bhairawaha" on the Nepali side but both are usually called Sunauli) is a small but busy town on the India-Nepal border and the most popular place for overland crossing between the two.
Sunauli has a extimated population of 70076. Sunauli is not a place to linger. It is an important border crossing point, but there are few services and zero sights - unless you count long lines of Indian big-rigs.
Have your paperwork ready before you arrive, Nepal visas are available on arrival but the "immigration office" (just a table) is not overly helpful.
Buses do not cross the border nor is there much in the way of any sort of "connecting service." Once you cross the border on foot you will need to find buses on the other side. Buses depart on both sides usually twice a day - dusk and dawn - but there is nothing like a real schedule.
There is only one main road to the town and what little there is within a few minutes walk of the border.
Most of the accommodation in town is geared towards truck drivers, not the nicest places and not recommended for women travelling alone. There is one government run hotel for pilgrims. Look for the large white building on the main road. Modern rooms with bathrooms, TVs, and A/C for 1200Rs. Cheaper rooms usually go early in the day.
Options for food on the Indian side of the border are very limited and of questionable standards (with the exception of the pilgrim's hotel). Eat on the Nepal side and pack extra food if possible. There is no market or groceries and even bananas or bread is hard to find.