sammeln
to collect
To deliberately gather things of a particular kind, often as a hobby or for a collection.
Mein Bruder sammelt seit seiner Kindheit Briefmarken.
My brother has been collecting stamps since his childhood.

to collect / gather
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to collect
To deliberately gather things of a particular kind, often as a hobby or for a collection.
Mein Bruder sammelt seit seiner Kindheit Briefmarken.
My brother has been collecting stamps since his childhood.

The verb 'sammeln' (to collect / gather) belongs to a word family. 'Die Sammlung' (collection) is the result of collecting, and 'der Sammler' or 'die Sammlerin' (collector) is the person who collects.
Er zeigte uns stolz seine wertvolle Kunstsammlung.
He proudly showed us his valuable art collection.
In the meaning 'sich konzentrieren' (to concentrate), 'sammeln' is always used reflexively: 'sich sammeln' (to collect oneself). The reflexive pronoun must be adapted to the subject.
Der Redner musste sich kurz sammeln, bevor er weitersprach.
The speaker had to collect himself briefly before continuing.
When 'sammeln' (to collect) describes a hobby, the objects are typically things from a particular category. Common examples are stamps, coins, postcards, or stickers.
Als Kind habe ich Servietten gesammelt.
As a child, I collected napkins.
You can collect not only physical objects, but also abstract things. Typical examples are 'Erfahrungen sammeln' (to gain experience), 'Informationen sammeln' (to gather information), or 'Kräfte sammeln' (to gather one's strength).
Nach dem Studium wollte sie erst einmal im Ausland Erfahrungen sammeln.
After university, she first wanted to gain experience abroad.
When money is collected for a specific purpose, you use the preposition 'für' followed by the accusative. This indicates the goal of the fundraising campaign.
Die Klasse sammelt für einen Ausflug in den Zoo.
The class is collecting for a trip to the zoo.
to donate
'sammeln' and 'spenden' often appear in the same context, for example with money or clothing for a good cause. The difference is important: someone who 'sammelt' brings together contributions from many people. Someone who 'spendet' gives something voluntarily themselves. It is easy to confuse the words because both happen during a campaign: a group collects money, and individual people donate money. With blood or organs, you normally say 'spenden', not 'sammeln'.

In der Schule sammeln wir Geld für ein neues Spielgerät.
At school, we are collecting money for a new piece of playground equipment.
Sie hat nach dem Gespräch im Büro fünfzig Euro gespendet.
After the conversation at the office, she donated fifty euros.