Your profile says what you are open to. That choice decides who employs you, who carries the risk when work stops, and which law protects you — so it is worth ten minutes once. Written by somebody who has placed people under all four.
You are self-employed. You invoice by the hour or the day, you carry your own insurance, your own holiday and your own gaps between contracts. What you owe is your working time and skill, competently applied — not a finished result. If the project fails despite good work, you are still paid.
What nobody tells you until it matters: Scheinselbständigkeit. If you work like an employee — fixed hours, their equipment, their instructions, one client for years — the Deutsche Rentenversicherung can decide you always were one. The bill for back social contributions goes to the client, and the client comes after you. Two things protect you: more than one client, and a contract describing a service rather than a seat.
§ 631 BGB. You promise a defined Werk — a thing that is finished or is not. Payment follows Abnahme (§ 640): the client accepts the work, and acceptance is the moment risk passes to them. Before it, the risk is yours.
What nobody tells you: you are liable for defects for two years after acceptance (§ 634a) — fixing them at your own cost, long after the invoice was paid. That is why a Werkvertrag should describe what "finished" means in enough detail that both sides would recognise it, and why an open-ended one is a trap rather than a convenience.
You are an employee — of the agency, not the client. Holiday pay, sick pay, notice, the employer's half of your social contributions: all of it applies, and it continues between assignments. The agency needs a licence to do this at all (AÜG).
Three things worth knowing. Equal Pay — after nine months you are entitled to what a comparable employee of the client earns, unless a collective agreement says otherwise, and Branchenzuschläge step you up before then. Überlassungshöchstdauer — eighteen months at one client, then the assignment must end whether or not either side wants it to; it is not negotiable between you and them. And your hourly wage is not what the client pays: the agency charges a factor on top, typically 1.8–2.5, which covers employing you, the bench, and the risk they carry.
The agency introduces you and is paid once, usually a share of your first year's salary. After that you and the company have the relationship and the agency is out of it. Notice periods, protection against dismissal after six months, and everything else in the BGB and KSchG applies normally.
Worth knowing: a placement fee is often tied to you staying — if you leave in the first months, the agency may have to repay it. That is between them and the client, but it explains why an agency asks how serious you are before putting you forward.
You employ yourself. You owe effort. You carry the gaps, the insurance and the Scheinselbständigkeit question.
You employ yourself. You owe a finished result, and two years of warranty after it is accepted.
The agency employs you. Equal Pay after nine months, eighteen months maximum at one client.
The client employs you. The agency is paid once and steps out.
A summary, not legal advice. The wording of your own contract decides which of these you are actually in — and it is the wording that counts, not the heading.