Es similar como palabras paralelas en español.

La forma como verbo, the verb form: to attach, juntar.

La forma como ajectivo, the adjectival form: attached, juntado.

La forma como sustantivo, the noun form: an attachment, la adjunción.

Attachment is a noun. You can have an attachment to something, just like you can have an opinion about something. Attached is a verb, so its usage is different. You can be attached to something, physically or emotionally, that then results in an attachment.

One of the places these two words are commonly used is in email.

Please see the attached documents

Please review the attachments

Another is in children’s attachment to their parents or caregivers.

He is very attached to his mother, he can’t be away from her.

She has an attachment disorder and doesn’t bond properly.

The last one complicates the noun/adjective picture because it’s a noun used as a adjective. In general attachment is a noun and attached is an adjective.

Thank you Luke!