For several years now, boosters have contained token cards representing the various token creatures that can be made by cards in that expansion. Many are fairly generic and have a name, type, and power and toughness. However, a few others do have more abilities such as flying, regeneration, deathtouch, lifelink and more.
You do not have to use these tokens–they are added to packs to make it easier and, for some people, more fun. When using tokens of any kind, there are two things that matter: both players need to be able to differentiate them and they need to be easily tapped and untapped.
With regards to being able to differentiate them, that means that you can tell what a token is how many of that type are in play. As mentioned above, you do not have to use the token cards that appear in packs; you can use empty sleeves of a color not being used by either player, tips and tricks cards, tokens made by others, or glass beads that are also used as counters. In fact, you can use the token cards to represent other tokens than what is written on them. For example, a 1/1 white flying bird token can be used if you do not have any 1/1 white flying spirit tokens available. All that matters is that both players know that these birds are actually spirits. When using tokens where the relevant text is not written on them, make sure to clearly announce what they are. When using a deck that make multiple types of tokens (I’m looking at you, G/W Humans), it can sometimes be useful to make space between the differing types.
The other thing that tokens need to be able to do is show when they are tapped or untapped. This is one way that rectangular cards are obviously useful. Coins or other completely round objects are not ideal, but they can be turned face up and face down to signify tapping. Glass beads, as mentioned above, can also be flipped like this to demonstrate their status. As above, the importance is clarity; clearly say to your opponent which tokens are tapped and how you plan to show which are tapped and which are not.