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Base64 is a way of representing binary data as plain ASCII text — it converts every 3 bytes of input into 4 printable characters drawn from a 64-character alphabet (A–Z, a–z, 0–9, + and /, with = used for padding). The encoded output is always about 33% larger than the input, which is the trade-off for making arbitrary data safe to transmit through text-only channels.
Embedding small images directly in HTML or CSS (data URIs), encoding the header and payload of a JWT token, attaching binary files in email (MIME), and pasting binary content into JSON, XML or other text-only formats that can't safely contain raw bytes. It's an encoding, not encryption — anyone can decode it back to the original data instantly, so it shouldn't be used to hide or protect sensitive information.
Base64 maps every 3 bytes (24 bits) of input to 4 characters (24 bits) of output, so the encoded form is always about 4/3 — roughly 33% — larger than the original. If the input length isn't a multiple of 3, one or two = padding characters are added at the end to keep the output length a multiple of 4.
Base64 encoding converts arbitrary binary data into a text-safe representation using 64 printable characters. Percent-encoding (URL encoding) instead replaces only the specific characters that are unsafe inside a URL, leaving the rest untouched. They solve different problems: use Base64 for embedding images or binary data in text formats; use URL encoding for values placed inside a URL or query string.