Impoverished parents are going to learn that welfare is a different bargain under the terms of an ambitious overhaul that becomes law Thursday and that is aimed at turning the Depression-era system into a gateway to the workforce.

For the first time, under the bill President Reagan is signing Thursday afternoon, states will be required to offer people on welfare a broad variety of education, training and work programs.Mothers of young children will be required to participate. They'll retain medical coverage for a year if they find a job. And they'll be far more likely to get child support payments.

All states will have to provide at least some cash benefits to families with unemployed fathers living at home. And non-custodial fathers will face new pressure for child support payments, with states required to identify more of them and automatically withhold payments from their wages.

Sen. Daniel Patrick Moynihan, D-N.Y., its chief architect, has called the bill "the most important legislation of its kind and the first legislation of its kind since the welfare system was established in the 1930s."

Even boosters concede there are many potential obstacles to the success of the Family Support Act: day care shortages, sagging economies in some states, work requirements that will cover only a fraction of welfare parents by 1995 - about 400,000 people in any given month, 20 percent of those who would be eligible.

But with $3.3 billion flowing to the states over a five-year phase-in period, they think they can make fundamental improvements in Aid to Families with Dependent Children, the existing federal-state welfare program serving 3.7 million families.

The new deal: the government will provide training and support services, and the parent must strive for independence.

"For the past 30 years, the job of the (state) welfare department has been to get the checks out to eligible families in the right amount on a timely basis," said Stephen Heintz, Connecticut's income maintenance commissioner and head of an American Public Welfare Association welfare reform task force.

"This legislation says that kind of income support system is not enough to help families overcome the many barriers they face," Heintz said. "The welfare system has to become a much more aggressive force."

Those most likely to feel the immediate effects of the welfare bill are able-bodied women with children age 3 and over. They are the prime targets of the new Job Opportunities and Basic Skills programs to be developed by each state.

While there already are a few small work programs for welfare mothers of children over 6, they operate on relatively little money and offer limited services in most states.

Under JOBS, by contrast, only parents of children under 3 are exempt, and states can lower that to age 1. Also, services must be much more extensive.

Parents will be offered basic and remedial education, literacy classes, job skills training, job readiness activities, job placement, child care and transportation. States must also provide two of the following: job search, on-the-job training, community work experience and work supplementation, in which a welfare grant subsidizes an individual's private wages.

Parents under 20 must earn a high school diploma or its equivalent.