While aarti songs and bhajan songs are both deeply associated with devotion and religion, the similarities end there. An aarti song, according to the Wikipedia website, is a song that is offered up in praise of the deity, as the lamps are being offered at the same time. On the other hand, a bhajan song is simply any sort of Indian devotional song.
For example, a bhajan could be as basic as either a kirtan or a mantra, yet it could also reach the sophisticated and more complicated reaches of either a kriti or even a dhrupad. In addition, the music in the more sophisticated forms of bhajan songs could be based upon classical talas or ragas.
It is also interesting to note that an aarti can refer to something else entirely and not just a song. Another meaning of aarti is a religious ritual of worship in the Hindu religion, where it is actually a portion of puja.
In Hinduism, puja is a broader religious ritual that is done in order to bring offerings to different Hindu deities, special guests or even just distinguished people.
In such a ritual, it is also common to witness light from wicks that have been soaked in either purified butter (ghee) or camphor (a white, waxy or transparent solid that comes complete with an aromatic and a strong odour) is offered up to various deities.
If you are in the market, currently, for either an aarti song or a bhajan song, you should check out the Sacred Music Guide website, which is a generous repository of these types of devotional and religious songs. On said website, you are welcome to search either by album or by track; perhaps you already know the aarti song or the bhajan song that you want to look for, or maybe you just have a vague idea of the kind of album that you want.
Either way, on the aforementioned website, you can find a good selection of aarti songs and also bhajan songs.
Aarti songs and bhajan songs are both used for devotional occasions, but, technically, they are different, to be sure. They are both songs that worshippers can utilise in order to get closer to the deity whom they are worshipping and honouring.