The northern beaches - Kaw Kwang, Klong Dao, and Phra Ae (Long Beach) - are all long, unblemished stretches of white sand and turquoise water, with the best conditions for swimming. As they are the closest to Saladan, the main entry point to the island, they also have the best (most developed) facilities, with a large choice of shops, dive schools, restaurants, bakeries and bars.
Kaw Kwang and Klong Dao have very gently sloping sand and are thus popular with families - especially those with small children, though Kaw Kwang is so shallow that at low tide you have to walk quite far out to get to the water. In general all the beaches on Ko Lanta are very tidal, meaning the water recedes quite far out during low tide (twice every 24 hours) and can cover up all but a few metres of sand at high tide (also twice every 24 hours).
Most of the resorts on these beaches have swimming pools, which also makes them attractive to the package holiday tourists. As you approach the southern tip of Klong Dao, around Holiday Villa, the beach gets narrower and quite rocky.
Phra Ae (also known as Long Beach) is Ko Lanta's 'trendy' beach, with a mix of boutique resorts and basic bungalows and is more popular with young people - who also like its lively nightlife scene. The beach itself is long and fairly characterless, but it is good for swimming.