Most girls from Laos go to find employment in Thailand because they simply can't find a job in Laos or they want more money, the monthly wage in Thailand is usually more than Laos. It's also cheaper for Thai employers to hire labourers from Laos so it's a win-win situation either way.
Most Laotians often get the impression that Lao girls who crosses the border to work in Thailand are usually involved in the Thai sex industry. This is not always true for everyone, but sometimes young Lao girls are imported for that purpose.
Lao girls in Thailand usually work in low-skilled jobs such as housemaids, in restaurants and factories. Work life in Thailand is not always easy, employers tend to be more strict than those in Laos and most of the time the girls are yelled at and forced to put alot of effort into something and work hard. That being said, the girls usually put up with their boss because all they can think of is the salary to support their families back home.
Sometimes Lao girls have relatives in Thailand who own a business so it's easy to go to work there with them, and therefore they are better treated and properly looked after than they would working for other people.
There are also Lao girls with a strong educational background who go to work in Thailand because they need special training and work related experiences. Companies in Thailand have better resources and offers higher pay for professional jobs.