tätig sein
to be employed
To work professionally; to be employed in a particular field or position. Often used in formal language.
Sie ist seit über zehn Jahren als Anwältin tätig.
She has been employed as a lawyer for over ten years.

active / employed
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to be employed
To work professionally; to be employed in a particular field or position. Often used in formal language.
Sie ist seit über zehn Jahren als Anwältin tätig.
She has been employed as a lawyer for over ten years.

The adjective 'tätig' (active/employed) is closely related to the noun 'die Tat' (action, deed). It therefore describes the state of carrying out an action or being active.
Er war in vielen sozialen Projekten ehrenamtlich tätig.
He was active as a volunteer in many social projects.
'Tätig' is used almost exclusively predicatively, meaning after a verb such as 'sein' (to be) or 'werden' (to become). Using it directly before a noun (e.g. 'ein tätiger Mann') is very rare and sounds outdated.
Mein Vater war 40 Jahre lang als Lehrer tätig.
My father worked as a teacher for 40 years.
The verb used changes the meaning: 'tätig sein' describes a state or profession, while 'tätig werden' describes the beginning of an action or an intervention.
Sie ist als Ärztin in einem Krankenhaus tätig.
She is employed as a doctor in a hospital.
The expression 'tätig sein' is a formal alternative to 'arbeiten als' (to work as). It is often used in CVs, official documents, and business language.
Dr. Meier ist in der Forschung und Entwicklung tätig.
Dr. Meier works in research and development.
to act / trade
'handeln' and 'tätig' can both mean that someone becomes active. 'handeln' is a verb and directly describes that someone does something, often because a situation requires a response. 'tätig' is an adjective, and in this meaning it usually appears in the phrase 'tätig werden' (to take action). That sounds somewhat more formal than simply 'handeln'. In everyday language, people often say 'wir müssen handeln' (we have to act); in authorities, workplaces, or official texts, you are more likely to hear 'jemand wird tätig' (someone takes action).

Nach der Beschwerde wurde die Hausverwaltung endlich tätig.
After the complaint, the property management company finally took action.
Wenn das Wasser in die Küche läuft, müssen wir sofort handeln.
If water is running into the kitchen, we have to act immediately.