SHA stands for "secure hashing algorithm".
SHA1 and SHA2 are the Signing algorithm and two different variations of the SHA algorithm.
The basic difference between SHA1 and SHA2 is the length of hash values generated by both algorithms.
SHA1 is a Single Hash function with 160-bit, while SHA2 is a Family of Hash Functions —224, 256, 384, and 512 it takes an input and produces a 160 bits hash value, while SHA2 takes an input and produces a 224,256,384 or 512 bits hash value.
SHA1 is less secure than SHA2, while SHA2 is more secure than SHA1, it is unreliable, while SHA2 is more reliable and has more improved certificates.
SHA1 generates a smaller hash compared to SHA2, while SHA2 generates a larger hash compared to SHA1.
Hash generated by SHA1 is weak, while Hash generated by SHA2 is strong.