A security agreement is a writing (or electronic record) that provides a description of the collateral, the debtor's intent to grant a security interest, and the debtor's signature.
The description must be sufficient to describe the particular item, as defined in UCC § 9-108.
In a majority of states, inventory, accounts receivable, chattel papers, etc. are assumed to include future inventory, etc. However, this should not be relied upon.
A financing statement can also be a security agreement.
A financing statement is a record that is publicly filed to put other people on notice of a security interest in the collateral.
A financing statement does not have to be very specific.
UCC § 9-108(c) says the very generic from the financing statement will not work.
A security can be oral. An authenticated security agreement cannot.