DSA and RSA Overview
The Digital Signature Algorithm (DSA) is an open standard used for creating digital signatures based on public key encryption. DSA is used in many different applications, including SSH and GPG, because it is an open standard and not subject to traditional copyright. The Rivest, Shamir, Adleman (RSA) algorithm is the first algorithm widely used to create digital signatures, but it is subject to copyright restrictions that some developers find onerous.
You will find that SSH uses RSA by default, whereas GPG uses DSA. As with many algorithms, you can specify various bit lengths; 1024 and 2048 are common lengths, but given the increase in processor speeds that permit ever-faster brute force attacks, 2048 is currently considered the minimal length to provide acceptable security.