1. The Classic HeartThe modern heart shape is a pictogram - an abstract symbol rather than a realistic drawing; while having some origin in the real form, it has lost much of its relationship to the object it represents - much as the 'smiley face' bears little relation to the human face, with simplified elements arranged in a meaningful pattern. Early representations of the heart were more rounded and reminiscent of the biological heart. Egyptian images of the heart were somewhat like a vase, with protruding handles somewhat suggestive of the large veins.
The forms that led to the modern heart shape are thought to have their origins in stylized ivy leaf shapes, which became visually cognate with the heart shape. But it's a complicated history - so much so that entire books have been written on the topic. The earliest known representation of the heart as a symbol of romantic love appears in a 13th century miniature, with a suitor offering his heart to a lady.
2. The Sacred HeartThe Sacred Heart, or Sacred Heart of Jesus, is a symbolic representation of the physical heart of Jesus, the Son of God in Christian tradition. and is usually encircled by an interwoven circle of thorny branches, representing the "Crown of Thorns" that was placed on Jesus' head, and may be surmounted with a symbolic cross (representing the Crucifiction), or a flame. The Sacred Heart may also be depicted as surrounded by beams of light or flames or both, suggesting the "transformative power" and "divine light" of love. It is frequently depicted in isolation, as part of a more elaborate design, or within a portrait of the Christ figure. If surrounded by flowers or pierced by seven swords, the heart may instead represent the Sacred Heart of Mary, the mother of Jesus in Christian mythology.
More about the meaning of the Sacred Heart