The most experienced weight lifters favor free weights such as fixed dumbbells, spinlock dumbbells and barbell weights. If building significant muscle mass is your goal then free weights are the best option for you.
Dumbbells are safer than barbells in that you can push yourself to failure on most exercises without having to worry about being crushed by 100 kilograms of weights, but remember its always best to be supervised when lifting weights.
A minor disadvantage of free weights is that certain exercises are harder to perform compared to using fixed weights on a multi gym for example. If you compare most free weight exercises to machine exercises you will always find free weights more challenging then using a machine, the reason is that free weights are literally free of movement which means they require more balance and coordination.
Another slight disadvantage of using gym weights is when it comes to working out your legs, because the amount of weight they can handle is usually more than is safe to lift without a partner to spot you.
Types of Weights & Bars
Standard Weights - Standard weight plates have a 1 inch diameter hole in the centre to insert a standard weightlifting bar, curl bar, triceps bar etc. The standard weights are recommended for beginners as they are generally less expensive in comparison to Olympic weights. Also remember the maximum weight limit on standard barbells is much less compared to Olympic weights. Typical standard bars have spin locks to secure the weight plates onto the barbell.
Olympic Weights - Olympic weights have a 2 inch diameter hole to insert your Olympic weightlifting bar. The weight lifting bar is also heavier (normally around 20kg) and the maximum weight the bar can hold is above 600kg. Olympic weights are recommended for intermediate to advanced weight lifters who are looking to lift heavier weights.