网上 (wǎng shang) means 'online' or 'on the internet' and is the everyday word for doing things on the internet (e.g., 在网上买东西). 线上 (xiàn shàng) is similar but often used to contrast with 线下 (offline), especially in contexts like shopping or events. Both are nouns, but 网上 is more general and common in daily speech.