Base64 encoding schemes are used when binary data needs to be stored or transferred as textual data. Therefore 64 characters are chosen that are both members of a subset common to most encodings (ASCII), and also printable.
Base64 is an encoding scheme used to represent binary data in an ASCII format. This is useful when binary data needs to be sent over media that are usually designed to handle textual data. Concrete examples would be sending images in an XML file or in an email attachment.
Bytes forming the data are broken into buffers of 24 bits (3 bytes at a time). The resulting buffer of 3 bytes is then brokenin 4 packs of 6 bits each. Those 6 bits form a number corresponding to the index in the character set supported by Base64 (A-Z, a-z, 0-9, + and /). If the number of bytes are not in numbers of three, then padding is used; == for 1 byte and = for 2 bytes.
Hell no! Base64 is not for encryption purposes and is not secure at all. It's an encoding mechanism, nothing more.