An employment based status is any immigration status someone holds specifically because of a job. It covers two very different things: employment-based nonimmigrant status — temporary work visas like H-1B, L-1, O-1, TN, and E-3, tied to a specific job and (in most categories) a sponsoring employer.

Employment based immigrant status is a green card, or permanent residence, obtained through employment.

It's a nonimmigrant work status for people coming to the U.S. in a "specialty occupation", a job that generally requires at least a U.S. bachelor's degree (or its equivalent) in a specific field. It's employer-sponsored, initially valid for up to 3 years (renewable to 6), and is one of the few statuses that allow "dual intent" (see below).

L-1 is a nonimmigrant work status for intracompany transferees moving to a related U.S. office of the same employer they've worked for abroad for at least one continuous year within the past three years. It's employer-sponsored and comes in two versions: L-1A for managers/executives (initially valid for up to 3 years, renewable to 7 total) and L-1B for specialized-knowledge employees (initially valid for up to 3 years, renewable to 5 total). Like the H-1B, L-1 status is one of the few statuses that allow "dual intent" (see below).

O-1 is a nonimmigrant work status for people with extraordinary ability or achievement in their field: O-1A covers sciences, business, education, and athletics; O-1B covers the arts and motion picture/TV work. It's employer- (or agent-) sponsored, initially valid for up to 3 years, and renewable in further increments for as long as the underlying work continues. O-1 isn't formally listed as a "dual intent" status the way H-1B and L-1 are. However, pursuing a green card while on O-1 generally isn't held against you.

TN is a nonimmigrant work status available only to Canadian and Mexican citizens under USMCA (f/k/a NAFTA), for jobs in a specific list of professional occupations set out in the agreement (things like engineers, accountants, scientists, and registered nurses). Because Canadian citizens do not require visa stamps in their passports, they can apply directly at the border, and it's initially valid for up to 3 years, renewable indefinitely in further 3-year increments. Mexican citizens require visa stamps, so they must apply at the US Consulate. Unlike H-1B, TN is not a dual intent status, so you have to maintain intent to depart at the end of your authorized stay, which can get complicated if you intend on pursuing a green card.

E-3 a nonimmigrant work status available only to Australian citizens, for a "specialty occupation" job much like H-1B, generally requiring at least a U.S. bachelor's degree (or its equivalent) in a specific field. It's employer-sponsored, initially valid for up to 2 years, and renewable indefinitely in further 2-year increments. Like the H-1B, it does have an annual numerical cap (of 10,500) but it has never actually been reached. Like TN, E-3 isn't a dual intent status, so pursuing a green card while on E-3 needs to be handled thoughtfully.

USCIS runs an annual electronic lottery each spring for people hoping to change into H-1B status from within the U.S., or to enter the U.S. on H-1B status. This is done because congress caps new H-1B visas at 85,000 a year: 65,000 a year for “regular cap”, or those with a Bachelors degree, and an additional 20,000 for those holding a U.S. master's degree or above. Only registrants who are randomly selected get the chance to actually file a full H-1B petition for that year.

Most H-1B petitions compete in the annual lottery because they're subject to the statutory cap of 65,000 (plus 20,000 for U.S. master's-degree holders) (See above). Petitions for jobs at universities, nonprofits affiliated with a university, or nonprofit or government research organizations are cap-exempt, meaning there is no lottery, and you can file any time of year.

Most nonimmigrant statuses legally require you to intend to return home. Dual intent is the exception that lets you simultaneously hold nonimmigrant status (like H-1B or L-1) while also pursuing a green card, without that green card process being held against your visa. F-1 and most other categories don't get this protection, which is why timing status changes around a green card filing matters more for those visas.

Both of these are for intracompany transferees moving to a related U.S. office of the same multinational employer, and both require having worked abroad for that company for at least one continuous year within the past three. L-1A is for managers and executives (up to 7 years total in status); L-1B is for employees with specialized knowledge of the company's proprietary processes or products (up to 5 years total).