While the term Arahant is sometimes used by Mahayana Buddhists, it is much more common in Theravada. This is mostly because Mahayana Buddhists generally aspire to be bodhisattvas, while Theravadins generally aspire to be Arahants.
The Sanskrit word is Arahant, the Pali word is Arhat. It is often seen misspelled as Arhant and Arahat. All four describe the same idea.
Theravada Buddhism distinguishes between three types of awakened (or enlightened) beings, all three having the same bliss of Nirvana (or Nibbana as Theravadins prefer to call it). Here are the distinctions as I have seen them in various Theravada sources:
Supreme Buddha - this is the type of Buddha we usually talk about when we use the term The Buddha, for example Gautama Buddha.
A Supreme Buddha is born very rarely, during a time of complete spiritual darkness when the memory of the previous Supreme Buddha has completely dissipated. He has prepared for this time during a number of rebirths as a bodhisatta.
However, when he is born as a human being, he does not consciously know all of dharma since he is raised as any other human during a period of spiritual darkness and ignorance. Nevertheless, thanks to his many lives as bodhisatta, he becomes fully enlightened. That is to say, he realizes the dharma in its fullness, and comes to complete understanding of the Four Noble Truths, including the Noble Eightfold Path.
After his enlightenment, he is fully emancipated and free from samsara. He then teaches the dharma to others who then can be equally emancipated and freed from samsara. Naturally, he only teaches those who wish to learn. It is impossible, even for the Supreme Buddha to emancipate anyone who does not wishes to be emancipated. He can only open the door, but he cannot make anyone walk through it.
Pacceka Buddha - this is a Pali word. In Sanskrit it is Pratyeka Buddha. He also is a Buddha, that is he becomes fully awakened/enlightened in the time of spiritual darkness. If I understand this correctly, the Pacceka Buddha, unlike the Supreme Buddha, does not teach others, whether because no one is ready to learn, or for any other reason.
Arahant - is the one, or one of many, who learns the dharma from the Supreme Buddha and subsequently becomes fully enlightened/awakened and realizes Nirvana. It does not have to be during the lifetime of the Supreme Buddha, but it always happens as a result of the study of what the Supreme Buddha taught.
Nevertheless, it is easiest to happen during the life of the Supreme Buddha. According to Theravada suttas it was not uncommon for thousands of people becoming Arahants in a single day, just shortly after encountering the Buddha and listening to him. However, many people continue becoming Arahants even centuries after the time of the Buddha.
Eventually, a dharma ending age ensues. During a dharma ending age, the memory of the latest Supreme Buddha and his teaching still exists, but fewer and fewer Arahants exist, as the understanding of the dharma is harder and harder.
Gautama Buddha predicted the next dharma ending age would start about 500 years after his time. That was 2,500 years ago...
So, what good is it to practice Buddhism today? Especially, if you are a Theravadin and aspire to become an Arahant? Well, first of all, it is still possible to become an Arahant today, it is just not as common. More importantly, practicing the Noble Eightfold Path is in meritorious karma which ensures a favorable rebirth, including the rebirth as a human when the next Supreme Buddha is born, so one can become an Arahant during that time.
Please understand that I am a Mahayana Buddhist and make all these write-ups about Theravada only because this is supposed to be Everything, and so far no Theravadin has posted here. To the best of my knowledge, however, this should be an acurate representation of Theravada teachings.
Also, my teacher gives a completely different definition of a Pratyeka (Pacceka) Buddha. According to him, a Pratyeka Buddha is one who selfishly desires Nirvana and comes to some level of enlightenment but is really not fully emancipated. I have not discussed this with other Mahayana teachers, so I do not know whether this is the generally accepted view of Mahayana. However, all Theravada literature that I have read describes the term Pacceka Buddha as I wrote about it above.