A bit is a single binary digit - which may be a 1 or a 0. When written in abbreviations, bits or things related to bits (like transfer speeds of network connections) are normally denoted by a lower case "b". A 100Mb connection is 100,000,000 bits.
A byte is also known as an octet. It is a group of 8 bits. When written in abbreviations, bytes or things related to bytes (like disk storage or memory capacity) are normally denoted by an upper case "B". A 500 GB (Gigabyte) hard drive is 500,000,000,000 bytes. If converted to an unsigned decimal, a byte can store a value from 0 to 255 or a single ASCII character.
Sometimes multiple bytes are grouped together (often 2, 4 or 8) and known as a word. Context is required to know the size of the word.
Large groups of bits or bytes are often noted with suffixes indicate a multiplier - like K for Kilo (1000) or Ki for Kebi (1024) - for more on this please our KB here.