The eIDAS Regulation defines a simple electronic signature as follows: "the data in electronic format attached to other electronic data or logically associated with them that the signatory uses to sign". The simple electronic signature is usually the one where you request to accept a condition by completing a checkbox or an OTP code and does not have the ability to properly identify the signer.
On its part, eIDAS Regulation defines the advanced electronic signature as: the electronic signature that meets the requirements set forth in article 26. The requirements contemplated in article 26 of the eIDAS Regulation are the following: i) it is uniquely linked to the signatory; ii) it is capable of identifying the signatory; iii) it is created using electronic signature creation data that the signatory can, with a high level of confidence, use under his sole control; and iv) it is linked to the data signed therewith in such a way that any subsequent change in the data is detectable. Unlike the simple electronic signature, through the advanced electronic signature, the signer has to draw his/her signature - properly speaking - with the finger or the mouse; and the signer is duly identified.
Although both electronic signatures have legal validity, the advanced electronic signature is more secure since it allows obtaining information aimed at properly identifying the signer and detecting any subsequent change. Therefore, it is recommended to use the simple electronic signature in those matters where it is not vital to properly identify the signer, while we recommend using the advanced electronic signature for all those documents or agreements that are sufficiently relevant and where it is necessary or useful enough to properly identify the signer.