Islam came to West Africa from the north and across the Sahara in the early part of the second millennium and quickly became a commanding influence with most of the local populace converting. Islam was brought to the Senegambia region by the Marabouts, religious figures traditionally from the Maghreb area of Northern Africa.
In the Islamic religion every believer has a direct relationship with Allah but because the societies of North and West Africa were so rigidly hierarchical it made more sense to have certain religious leaders all ascribed with divine power providing access to the godhead. Brotherhoods would then grow up around certain Marabouts, many of whom would come to be revered as saints. The Qadiriya brotherhood was one of the first to reach Senegal and spread across the country in the 19th century and is still widely followed today, mostly by the Mandinka people. It is true that in many cases the term has lost some of its historical meaning and is now applied to anyone who leads a certain kind of devotedly Islamic life, so you will see Marabouts all over Senegal, clothed in traditional dress robes and leading a spare, ascetic life.
There are many hundreds of localised religions in West Africa and these are generally centred around an idea of animism, or that any animal, plant or object has a soul or spirit and should be treated as such. This can mean that certain areas or places are considered sacred and possessed of spirits – or indeed by the souls of ancestors - and it is common to see offerings to these spirits (sometimes even called deities) left in the form of incense or flowers.