Digital Signature Certificates (DSC) are the digital equivalent (that is electronic format) of physical or paper certificates. Examples of physical certificates are drivers licenses, passports or membership cards. Certificates serve as proof of identity of an individual for a certain purpose; for example, a drivers license identifies someone who can legally drive in a particular country. Likewise, a digital certificate can be presented electronically to prove your identity, to access information or services on the Internet or to sign certain documents digitally.

The various uses are outlined here.

a) Your Digital Signature Certificate could be used to allow you to access membership-based web sites automatically without entering a user name and password. It can allow others to verify your "signed" e-mail or other electronic documents, assuring your intended reader(s) that you are the genuine author of the documents, and that the content has not been corrupted or tampered with in any way.

b) Digital Signature Certificates enables others to send private messages to you: anyone else who gets his/her hands on a message meant for you will not be able to read it.

c) For filings your income tax returns.

d) For applying Importer exporter code number. 

Like physical documents are signed manually, electronic documents, for example e-forms are required to be signed digitally using a Digital Signature Certificate. It can allow others to verify your "signed" electronic documents, assuring your intended reader(s) that you are the genuine author of the documents, and that the content has not been corrupted or tampered with in any way.

Digital Signature Certificate (DSC) is not required by LLP but by individuals.  The Designated Partner of LLP, signing on behalf of the LLP  will require a DSC.

No.

No. It is mandatory to have a valid digital signature certificate(DSC) for e-filing the forms on the LLP portal.

You should keep the media carrying your digital signature safely and not disclose your password to anybody.

Digital signatures are based on Public Key Infrastructure (PKI) and are a result of a cryptographic operation that guarantees signer authenticity, data integrity and non-repudiation of signed documents. The digital signature cannot be copied, tampered or altered. In addition, because they are based on standard PKI technology, signatures made within one application (e.g. Microsoft Word, Adobe PDF) can be validated by others using the same applications. On the other hand, an electronic signature is a proprietary format (there is no standard for electronic signatures) that is an electronic data, such as a digitized image of a handwritten signature, a symbol, voiceprint, etc., that identifies the author(s) of an electronic message. Electronic signatures are vulnerable to copying and tampering, making forgery easy. In many cases, they are not legally binding and will require proprietary software to validate the signature.

A licensed Certifying Authority (CA) issues the digital signature. Certifying Authority (CA) means a person who has been granted a license to issue a digital signature certificate under Section 24 of the Indian IT-Act 2000.
The list of licensed CAs along with their contact information is available on the www.mca.gov.in.

A CA is a trusted third party willing to verify the ID of entities and their association with a given key, and later issue certificates attesting to that identity. In the passport analogy, the CA is similar to the Ministry of external affairs, which verifies your identification, creates a recognized and trusted document which certifies who you are, and issues the document to you.

A CA can be within the organization itself or outside organization depending on the purpose of the certificates. A company may issue certificates to its employees for reason that only its employees can access to the company database but an internet user might request for a certificate from a well-known and trusted CA in order for him to do on-line transaction securely.

The different types of Digital Signature Certificates are: 
Class 1: Generally used for demo purpose, does not have high security features.
Class 2: Here, the identity of a person is verified against a trusted, pre-verified database.
Class 3: This is the highest level where the person needs to present himself or herself in front of a Registration Authority (RA) and prove his/ her identity.

DSC of Class 2 and Class 3 category issued by a licensed Certifying Authority (CA) needs to be obtained for efiling on the MCA portal.
                                                             

The time taken by CAs to issue a DSC may vary from three to seven days.

Digital Signatures are legally admissible in a Court of Law,  as provided under the provisions of Information Technology Act 2000

No, only Digital Signatures issued by companies  in India which has been authorized by Controller of Certifying Authority of India is acceptable.

Following documents are required: 
Indian National Identity Proof  
Present Residential Proofs  
One Photo  
Certificate from Bank verifying the signatures or certificate from Indin            Company, if any, in which he is a Director. 

Note:
 The Identity / Residential Proofs are need to notarized by a notary in their country, in case of foreign National.
  Proofs like Electricity Bill, Phone Bill, Bank Statement should not be more than two months old.