The differences between any two denominations vary. There are many differences between Baptist and Pentecostal denominations. The biggest one is speaking in tongues. Baptist churches do not believe that the Baptism of the Holy Ghost is apart of the plan of salvation. Pentecostal churches teach that the Holy Ghost is an essential part of being a Christian.
I am a Pentecostal (Assemblies of God) believer, who attended a Baptist church in my younger years.
The
majority of Pentecostal denominations practice believer's baptism - i.e., baptism of professing believers who are old enough to understand the significance of what they are doing. This is the same as the Baptist position.
There are
some Pentecostal churches that baptise infants (such as the Pentecostal Methodist Church of Chile), but they are in the minority.
I just would like to clarify this answer. Oneness Pentecostal churches baptize "in the name of Jesus" and Baptist churches will baptize "in the name of the Father, the Son, and the Holy Spirit". Another difference is that Pentecostals believe in baptism by the Holy Spirit where one speaks in tongues, and Baptists believe that the Holy Spirit enters at the moment one is saved.