Caulking

There are lots of types of caulk , which has an elastic characteristic. The tubes that require a caulking gun, and the smaller hand squeezed ones. For painting always buy the paintable latex caulk. The 100% silicon caulks cannot be painted and will cause whoever comes later to paint your handy work a lot of grief.
Apply caulk by cutting a small amount off the end of the tube,then with a steady stroke squeeze the caulk into all the area's between door casing, windows etc that are separating. (Some scraping maybe required before this step.) Then, with a damp rag draw your finger along the caulk to wipe off the excess. If you have a mound of caulk on the rag you are applying too much. Rinse the rag often and remember you cannot sand away the excess later so remove all the excess caulk.