Fricatives are consonants that when they are produced the air escapes through a small passage and makes a hissing sound. There are fricatives in all languages and they are considered continuant consonants. That means someone can make them continuously as long as there is air in the lungs (plosives are not continuant).
As a quick example, make a long, hissing /s/ sound and gradually lower your tongue so that it is no longer close to the roof of the mouth. The hissing sound will stop as the air passage gets larger.
Affricates are complex consonants. They begins as plosives and end as fricatives. A simple example is the affricate sound heard at the end of the word "search" (/sɜrtʃ/). It begins with an articulation practically the same as the closure and hold presses of "t", but instead of rapid release with plosion and aspiration, as we can find at the end of the word "sort" (/sɔrt/), the tongue moves to the position for the fricative /ʃ/ as we can find in the word "shop" (/ʃɒp/).
The result: the plosive is followed immediately by a fricative noise. Since this sound is composed of /t/ and /ʃ/ it is represented as /tʃ/.
However, not all plosive plus fricative sequences are classified as affricates. For example the plosive /k/ followed by the fricative /f/ (as in the middle of the word "breakfast" /ˈbrɛk fəst/) is not considered affricate since it is not made with the same articulators; which means they are not homorganic. But /t/ and /ʃ/ are both made with the tongue blade against the alveolar ridge so they are homorganic.